core: Clean up includes

Split out of #37344.

src/core/exec-credential.h

@@ -1,14 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-#include <stdbool.h>
-#include <stdint.h>
-#include <unistd.h>
-
-#include "hash-funcs.h"
-
-typedef struct ExecContext ExecContext;
-typedef struct ExecParameters ExecParameters;
+#include "core-forward.h"
 
 /* A credential configured with LoadCredential= */
 typedef struct ExecLoadCredential {