x11/way-displays: unbreak on -CURRENT after e8fe34cde858
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