Fix compilation under GCC 10+

[h3xx]

Fixes an error during linking. E.g.:

Error log snippet

cd /usr/src/git/pcsxr/_build/plugins/dfnet && /usr/bin/cmake -E cmake_link_script CMakeFiles/DFNet.dir/link.txt --verbose=1
/usr/bin/cc -fPIC -O3 -DNDEBUG -s -shared  -o libDFNet.so CMakeFiles/DFNet.dir/cfg.c.o CMakeFiles/DFNet.dir/dfnet.c.o CMakeFiles/DFNet.dir/unix.c.o
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x0): multiple definition of `wset'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x0): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x80): multiple definition of `rset'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x80): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x120): multiple definition of `conf'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x120): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x104): multiple definition of `PadSize'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x104): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x106): multiple definition of `PadRecvSize'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x106): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x107): multiple definition of `PadSendSize'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x107): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x110): multiple definition of `PadSendData'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x110): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x100): multiple definition of `WaitCancel'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x100): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/dfnet.c.o:(.bss+0x108): multiple definition of `PadRecvData'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x108): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0x120): multiple definition of `WaitCancel'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x100): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0x140): multiple definition of `conf'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x120): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0x20): multiple definition of `wset'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x0): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0xa0): multiple definition of `rset'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x80): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0x124): multiple definition of `PadSize'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x104): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0x126): multiple definition of `PadRecvSize'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x106): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0x127): multiple definition of `PadSendSize'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x107): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0x128): multiple definition of `PadRecvData'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x108): first defined here
/usr/bin/ld: CMakeFiles/DFNet.dir/unix.c.o:(.bss+0x130): multiple definition of `PadSendData'; CMakeFiles/DFNet.dir/cfg.c.o:(.bss+0x110): first defined here
collect2: error: ld returned 1 exit status
make[2]: *** [plugins/dfnet/CMakeFiles/DFNet.dir/build.make:132: plugins/dfnet/libDFNet.so] Error 1
make[2]: Leaving directory '/usr/src/git/pcsxr/_build'
make[1]: *** [CMakeFiles/Makefile2:726: plugins/dfnet/CMakeFiles/DFNet.dir/all] Error 2
make[1]: Leaving directory '/usr/src/git/pcsxr/_build'
make: *** [Makefile:139: all] Error 2
make: Leaving directory '/usr/src/git/pcsxr/_build'

Reason for WIP: I still want to test this on my local machine. Good to go.