don't warn when file descriptor equality is unknown

linux uses kcmp(2) with KCMP_FILE, we have no equivalent

lib/mesa/src/gallium/winsys/amdgpu/drm/amdgpu_winsys.c

@@ -397,10 +397,12 @@ are_file_descriptions_equal(int fd1, int fd2)
       static bool logged;
 
       if (!logged) {
+#if DETECT_OS_LINUX
          os_log_message("amdgpu: os_same_file_description couldn't "
                         "determine if two DRM fds reference the same "
                         "file description.\n"
                         "If they do, bad things may happen!\n");
+#endif
          logged = true;
       }
    }