Cast uses of return_*(), nop() and enumerator_create_empty()

As described in the previous commit, GCC 15 uses C23 by default and that
changes the meaning of such argument-less function declarations.  So
whenever we assign such a function to a pointer that expects a function
with arguments it causes an incompatible pointer type warning.  We
could define dedicated functions/callbacks whenever necessary, but this
seems like the simpler approach for now (especially since most uses of
these functions have already been cast).

src/libcharon/plugins/updown/updown_handler.c

@@ -220,7 +220,7 @@ updown_handler_t *updown_handler_create()
 			.handler = {
 				.handle = _handle,
 				.release = _release,
-				.create_attribute_enumerator = enumerator_create_empty,
+				.create_attribute_enumerator = (void*)enumerator_create_empty,
 			},
 			.create_dns_enumerator = _create_dns_enumerator,
 			.destroy = _destroy,