Explicitly depend on gio-unix-2.0

This fixes the following build failure on NixOS:

xfsm-client.c:32:10: fatal error: gio/gdesktopappinfo.h: No such file or directory
   32 | #include <gio/gdesktopappinfo.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~

https://docs.gtk.org/gio/class.DesktopAppInfo.html

This works on many distros because their pkg-config --cflags returns Cflags from Requires.private, so this is added via gtk+-3.0. On NixOS however, we have decided to interpret the pkg-config specification to not include Requires.private in --cflags to avoid https://gitlab.freedesktop.org/pkg-config/pkg-config/-/issues/7 and https://gitlab.freedesktop.org/pkg-config/pkg-config/-/issues/28.

But no matter how, it is always a good idea to directly depend on all libraries whose headers we include. Transitive dependencies may be in theory removed any time so relying on them might be unsafe.