Fix FindBpfObject.cmake bpf object header search setup

Quote of `CLANG_SYSTEM_INCLUDES` should be removed for
`add_custom_command`, otherwise all the options in this variable will be
treated as a single string and it causes header searching paths not
correctly set:

```
[ 56%] [clang] Building BPF object: minimal
In file included from /home/debian/projects/libbpf-bootstrap/examples/c/minimal.bpf.c:3:
In file included from /usr/include/linux/bpf.h:11:
/usr/include/linux/types.h:5:10: fatal error: 'asm/types.h' file not found
         ^~~~~~~~~~~~~
1 error generated.
make[2]: *** [CMakeFiles/minimal.dir/build.make:78: minimal.bpf.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:288: CMakeFiles/minimal.dir/all] Error 2
make: *** [Makefile:91: all] Error 2
```

Signed-off-by: Xin Zhang <zhangxin.xa@gmail.com>

Author: xzhangxa

tools/cmake/FindBpfObject.cmake

@@ -133,7 +133,7 @@ execute_process(
   RESULT_VARIABLE CLANG_SYSTEM_INCLUDES_result
   OUTPUT_STRIP_TRAILING_WHITESPACE)
 if(${CLANG_SYSTEM_INCLUDES_result} EQUAL 0)
-  string(REPLACE "\n" " " CLANG_SYSTEM_INCLUDES ${CLANG_SYSTEM_INCLUDES_output})
+  separate_arguments(CLANG_SYSTEM_INCLUDES UNIX_COMMAND ${CLANG_SYSTEM_INCLUDES_output})
   message(STATUS "BPF system include flags: ${CLANG_SYSTEM_INCLUDES}")
 else()
   message(FATAL_ERROR "Failed to determine BPF system includes: ${CLANG_SYSTEM_INCLUDES_error}")
@@ -165,6 +165,7 @@ macro(bpf_object name input)
     COMMAND ${BPFOBJECT_CLANG_EXE} -g -O2 -target bpf -D__TARGET_ARCH_${ARCH}
             ${CLANG_SYSTEM_INCLUDES} -I${GENERATED_VMLINUX_DIR}
             -isystem ${LIBBPF_INCLUDE_DIRS} -c ${BPF_C_FILE} -o ${BPF_O_FILE}
+    COMMAND_EXPAND_LISTS
     VERBATIM
     DEPENDS ${BPF_C_FILE}
     COMMENT "[clang] Building BPF object: ${name}")