Fix include files for fork()

Author: AMDmi3

CMakeLists.txt

@@ -294,14 +294,14 @@ if(SDL2MIXER_CMD)
     target_compile_definitions(SDL2_mixer PRIVATE MUSIC_CMD)
     set(fork_found OFF)
     if(NOT fork_found)
-        check_symbol_exists(fork sys/unistd.h HAVE_FORK)
+        check_symbol_exists(fork unistd.h HAVE_FORK)
         if(HAVE_FORK)
             set(fork_found ON)
             target_compile_definitions(SDL2_mixer PRIVATE HAVE_FORK)
         endif()
     endif()
     if(NOT fork_found)
-        check_symbol_exists(fork sys/unistd.h HAVE_VFORK)
+        check_symbol_exists(fork unistd.h HAVE_VFORK)
         if(HAVE_VFORK)
             set(fork_found ON)
             target_compile_definitions(SDL2_mixer PRIVATE HAVE_VFORK)