core: fix build error due to merge conflict in varlink-execute.h

During last refactor, an include wasn't changed and led to a build
error.

Follow-up for fdb2c0dd6f

src/core/varlink-execute.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include "forward.h"
+#include "shared-forward.h"
 
 int unit_exec_context_build_json(sd_json_variant **ret, const char *name, void *userdata);