Fix stack variable scope from previous commit.

The defect came from me, not from that commit's credited author.  Per
buildfarm members olingo and grassquit.

Discussion: https://postgr.es/m/20240903192030.1e@rfd.leadboat.com

Author: nmisch

contrib/pg_visibility/pg_visibility.c

@@ -490,6 +490,7 @@ collect_visibility_data(Oid relid, bool include_pd)
 	BlockNumber blkno;
 	Buffer		vmbuffer = InvalidBuffer;
 	BufferAccessStrategy bstrategy = GetAccessStrategy(BAS_BULKREAD);
+	BlockRangeReadStreamPrivate p;
 	ReadStream *stream = NULL;
 
 	rel = relation_open(relid, AccessShareLock);
@@ -505,8 +506,6 @@ collect_visibility_data(Oid relid, bool include_pd)
 	/* Create a stream if reading main fork. */
 	if (include_pd)
 	{
-		BlockRangeReadStreamPrivate p;
-
 		p.current_blocknum = 0;
 		p.last_exclusive = nblocks;
 		stream = read_stream_begin_relation(READ_STREAM_FULL,