exo: Explicitly add GIO_UNIX_CFLAGS
Fixes the following on NixOS:
exo-execute.c:25:10: fatal error: gio/gdesktopappinfo.h: No such file or directory
25 | #include <gio/gdesktopappinfo.h>
| ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
The reason here is the same as xfce4-session!51 (merged).
Here XDT_CHECK_OPTIONAL_PACKAGE
does manage to find gio-unix-2.0, so HAVE_GIO_UNIX
is defined,
it is just the cflags missing for us.