src: shared: plugin: add missing includes to header

Needed for clang-tidy prep work, as it requires headers to work standalone
(which is useful anyway).

src/shared/plugin.h

@@ -18,6 +18,11 @@
 #ifndef __LIBRC_PLUGIN_H__
 #define __LIBRC_PLUGIN_H__
 
+#include <stdbool.h>
+#include <sys/types.h>
+
+#include "rc.h"
+
 /* A simple flag to say if we're in a plugin process or not.
  * Mainly used in atexit code. */
 extern bool rc_in_plugin;