core: Clean up includes

Split out of #37344.

src/core/exec-invoke.h

@@ -1,11 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-typedef struct ExecCommand ExecCommand;
-typedef struct ExecContext ExecContext;
-typedef struct ExecParameters ExecParameters;
-typedef struct ExecRuntime ExecRuntime;
-typedef struct CGroupContext CGroupContext;
+#include "core-forward.h"
 
 int exec_invoke(
                 const ExecCommand *command,