Header comment of shm_mq.c was mistakenly specifying path to shm_mq.h.
It was introduced in
ec9037df. So, theoretically it could be
backpatched to 9.4, but it doesn't seem to worth it.
* Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
- * src/include/storage/shm_mq.h
+ * src/backend/storage/ipc/shm_mq.c
*
*-------------------------------------------------------------------------
*/