build: Prototype GType accessors for private classes

Otherwise we fail to build with -Werror=missing-prototypes.

https://bugzilla.gnome.org/show_bug.cgi?id=687385

gio/tests/gdbus-peer-object-manager.c

@@ -39,6 +39,7 @@ typedef struct {
   GDBusInterfaceSkeletonClass parent_class;
 } MockInterfaceClass;
 
+static GType mock_interface_get_type (void);
 G_DEFINE_TYPE (MockInterface, mock_interface, G_TYPE_DBUS_INTERFACE_SKELETON);
 
 static void