Fix imgui not building on distros which don't have wayland headers on the default include path on Linux

[helaslo]

Commit 4457dd5 updated imgui to a version which pulls in GLFW's wayland implementation in a mandatory way, which requires wayland-client headers.

This builds fine on most distros, however some like openSUSE Tumbleweed (and presumably Leap too, haven't checked) have wayland-client.h in a subdirectory, not in the main include path, which usally is /usr/include.

See output of RPM to show the directory structure on Tumbleweed:

rpm -ql wayland-devel
/usr/bin/wayland-scanner
/usr/include/wayland
/usr/include/wayland/wayland-client-core.h
/usr/include/wayland/wayland-client-protocol.h
/usr/include/wayland/wayland-client.h
/usr/include/wayland/wayland-cursor.h
/usr/include/wayland/wayland-egl-backend.h
/usr/include/wayland/wayland-egl-core.h
/usr/include/wayland/wayland-egl.h
/usr/include/wayland/wayland-server-core.h
/usr/include/wayland/wayland-server-protocol.h
/usr/include/wayland/wayland-server.h
/usr/include/wayland/wayland-util.h
/usr/include/wayland/wayland-version.h
/usr/lib64/libwayland-client.so
/usr/lib64/libwayland-cursor.so
/usr/lib64/libwayland-egl.so
/usr/lib64/libwayland-server.so
/usr/lib64/pkgconfig/wayland-client.pc
/usr/lib64/pkgconfig/wayland-cursor.pc
/usr/lib64/pkgconfig/wayland-egl-backend.pc
/usr/lib64/pkgconfig/wayland-egl.pc
/usr/lib64/pkgconfig/wayland-scanner.pc
/usr/lib64/pkgconfig/wayland-server.pc
/usr/share/aclocal
/usr/share/aclocal/wayland-scanner.m4
/usr/share/wayland
/usr/share/wayland/wayland-scanner.mk
/usr/share/wayland/wayland.dtd
/usr/share/wayland/wayland.xml

This change builds fine on all Linux distros in CI, wayland-client libraries are pulled in implicitly already, but if needed I can update the docs to include installation of this library