socketlistener: fix event signature in vfunc

The GSocketListener::event signal has a type of GSocketListenerEvent,
which is an enum. However, the vfunc signature had a pointer, with
different sizing requirements. Given the alignment and prompostion
of some systems, you may still get the same call-site layout, but
that is not guaranteed.

This fixes the parameter to have the proper enumeration.

https://bugzilla.gnome.org/show_bug.cgi?id=791906

gio/gsocketlistener.h

@@ -62,7 +62,7 @@ struct _GSocketListenerClass
   void (* changed) (GSocketListener *listener);
 
   void (* event) (GSocketListener      *listener,
-                  GSocketListenerEvent *event,
+                  GSocketListenerEvent  event,
                   GSocket              *socket);
 
   /* Padding for future expansion */