Print lwlock stats also for aux processes, when built with LWLOCK_STATS

InitAuxiliaryProcess() closely resembles InitProcess(), but it didn't
call InitLWLockAccess(). But because InitLWLockAccess() is a no-op
unless compiled with LWLOCK_STATS, and everything works even if it's
not called, the only consequence was that the stats were not printed
for aux processes.

This was an oversight in commit 1c6821be31, in version 9.5, so it is
missing in all supported branches. But since it only affects
developers using LWLOCK_STATS and no one has complained, no
backpatching.

Discussion: https://www.postgresql.org/message-id/20231130202648.7k6agmuizdilufnv@awork3.anarazel.de

src/backend/storage/lmgr/proc.c

@@ -614,6 +614,13 @@ InitAuxiliaryProcess(void)
 	 * Arrange to clean up at process exit.
 	 */
 	on_shmem_exit(AuxiliaryProcKill, Int32GetDatum(proctype));
+
+	/*
+	 * Now that we have a PGPROC, we could try to acquire lightweight locks.
+	 * Initialize local state needed for them.  (Heavyweight locks cannot be
+	 * acquired in aux processes.)
+	 */
+	InitLWLockAccess();
 }
 
 /*