core: remove value for first #define in headers, add "PLUGIN" in plugin headers

src/plugins/exec/exec.h

@@ -17,8 +17,8 @@
  * along with WeeChat.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef WEECHAT_EXEC_H
-#define WEECHAT_EXEC_H 1
+#ifndef WEECHAT_PLUGIN_EXEC_H
+#define WEECHAT_PLUGIN_EXEC_H
 
 #include <unistd.h>
 #include <time.h>
@@ -88,4 +88,4 @@ extern int exec_process_cb (const void *pointer, void *data,
 extern void exec_free (struct t_exec_cmd *exec_cmd);
 extern void exec_free_all ();
 
-#endif /* WEECHAT_EXEC_H */
+#endif /* WEECHAT_PLUGIN_EXEC_H */