pg_upgrade: remove C99 compiler req. from commit 3c0471b5fd

This commit required support for inline variable definition, which is
not a requirement.

RELEASE NOTE AUTHOR:  the author of commit 3c0471b5fd
(pg_upgrade/tablespaces) was Justin Pryzby, not me.

Reported-by: Andres Freund

Discussion: https://postgr.es/m/20201016001959.h24fkywfubkv2pc5@alap3.anarazel.de

Backpatch-through: 9.5

src/bin/pg_upgrade/check.c

@@ -547,11 +547,12 @@ create_script_for_cluster_analyze(char **analyze_script_file_name)
 static void
 check_for_new_tablespace_dir(ClusterInfo *new_cluster)
 {
+	int		tblnum;
 	char	new_tablespace_dir[MAXPGPATH];
 
 	prep_status("Checking for new cluster tablespace directories");
 
-	for (int tblnum = 0; tblnum < os_info.num_old_tablespaces; tblnum++)
+	for (tblnum = 0; tblnum < os_info.num_old_tablespaces; tblnum++)
 	{
 		struct stat statbuf;