x11/way-displays: unbreak on -CURRENT after e8fe34cd

Add -isystem to hide warnings in headers unintentionally exposed via symlink.

In file included from src/fds.c:6:
/usr/local/include/sys/inotify.h:21:13: error: zero size arrays are an extension [-Werror,-Wzero-length-array]
   21 |         char            name[0];
      |                              ^

PR:		288342
Reported by:	pkg-fallout