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

gobject/tests/param.c

@@ -457,6 +457,7 @@ static void test_object_c_init (TestObjectC *c) { }
  * That gives 21 (7 * 3) properties that will be installed.
  */
 typedef GTypeInterface TestInterfaceInterface;
+static GType test_interface_get_type (void);
 //typedef struct _TestInterface TestInterface;
 G_DEFINE_INTERFACE (TestInterface, test_interface, G_TYPE_OBJECT)
 static void