Cast iface_init to GInterfaceInitFunc

This is in line with what all other type define macros do.

https://bugzilla.gnome.org/show_bug.cgi?id=508157

tests/gobject/dynamictype.c

@@ -93,8 +93,7 @@ dynamic_object_class_finalize (DynamicObjectClass *class)
 }
 
 static void
-dynamic_object_iface_init (gpointer g_iface,
-			   gpointer iface_data)
+dynamic_object_iface_init (TestIface *iface)
 {
 }