Don't provide the SDL3 header path

The recommended way to include SDL.h is #include <SDL3/SDL.h>, and having both paths available was confusing for people trying to port to SDL3.

Thanks for the feedback!
https://github.com/exult/exult/issues/379#issuecomment-1701066992

cmake/sdl3.pc.in

@@ -10,4 +10,4 @@ Version: @PROJECT_VERSION@
 Requires.private: @SDL_PC_PRIVATE_REQUIRES@
 Conflicts:
 Libs: -L${libdir} @SDL_RLD_FLAGS@ @SDL_PC_LIBS@ @SDL_PC_SECTION_LIBS_PRIVATE@ @SDL_PC_STATIC_LIBS@
-Cflags: -I${includedir} -I${includedir}/SDL3 @SDL_PC_CFLAGS@
+Cflags: -I${includedir} @SDL_PC_CFLAGS@