Require gio-unix-2.0 instead of gio-2.0
This fixes the following on NixOS:
AppInfos.hpp:13:10: fatal error: gio/gdesktopappinfo.h: No such file or directory
13 | #include <gio/gdesktopappinfo.h>
| ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
The reason here is the same as xfce/xfce4-session!51 (merged).
It looks only gio/gdesktopappinfo.h
is used here.