common: Explicitly depend on gio-unix-2.0

This fixes the following on NixOS:

xfce-notify-log-util.c:40:10: fatal error: gio/gdesktopappinfo.h: No such file or directory
   40 | #include <gio/gdesktopappinfo.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

The reason here is the same as https://gitlab.xfce.org/xfce/xfce4-session/-/merge_requests/51.

common/Makefile.am

@@ -19,13 +19,15 @@ libxfce_notifyd_common_la_SOURCES = \
 libxfce_notifyd_common_la_CFLAGS = \
 	$(LIBXFCE4UI_CFLAGS) \
 	$(GTK3_CFLAGS) \
+	$(GIO_UNIX_CFLAGS) \
 	$(SQLITE_CFLAGS) \
 	$(XFCONF_CFLAGS) \
 	$(PLATFORM_CFLAGS)
 
 libxfce_notifyd_common_la_LIBADD = \
 	$(LIBXFCE4UI_LIBS) \
 	$(GTK3_LIBS) \
+	$(GIO_UNIX_LIBS) \
 	$(XFCONF_LIBS) \
 	$(SQLITE_LIBS)