Updated gd_glue.cpp to work with the latest changes in the variant refactoring

Without this change the engine dont compile with the mono module enabled.

modules/mono/glue/gd_glue.cpp

@@ -55,7 +55,8 @@ MonoObject *godot_icall_GD_convert(MonoObject *p_what, int32_t p_type) {
 	Variant what = GDMonoMarshal::mono_object_to_variant(p_what);
 	const Variant *args[1] = { &what };
 	Callable::CallError ce;
-	Variant ret = Variant::construct(Variant::Type(p_type), args, 1, ce);
+	Variant ret;
+	Variant::construct(Variant::Type(p_type), ret, args, 1, ce);
 	ERR_FAIL_COND_V(ce.error != Callable::CallError::CALL_OK, nullptr);
 	return GDMonoMarshal::variant_to_mono_object(ret);
 }