Add missing shmem size estimate for fast-path locking struct

It's been missing ever since fast-path locking was introduced. It's a
small discrepancy, about 4 kB, but let's be tidy. This doesn't seem
worth backpatching, however; in stable branches we were less precise
about the estimates and e.g. added a 10% margin to the hash table
estimates, which is usually much bigger than this discrepancy.

src/backend/storage/lmgr/lock.c

@@ -3775,6 +3775,9 @@ LockManagerShmemSize(void)
 	max_table_size *= 2;
 	size = add_size(size, hash_estimate_size(max_table_size, sizeof(PROCLOCK)));
 
+	/* fast-path structures */
+	size = add_size(size, sizeof(FastPathStrongRelationLockData));
+
 	return size;
 }