pg_prewarm: Add missing LWLockRegisterTranche call.

Commit 79ccd7cbd5, which added automatic
prewarming, neglected this.

Kyotaro Horiguchi, reviewed by me.

Discussion: http://postgr.es/m/20171215.173219.38055760.horiguchi.kyotaro@lab.ntt.co.jp

contrib/pg_prewarm/autoprewarm.c

@@ -767,6 +767,8 @@ apw_init_shmem(void)
 	}
 	LWLockRelease(AddinShmemInitLock);
 
+	LWLockRegisterTranche(apw_state->lock.tranche, "autoprewarm");
+
 	return found;
 }