psql \dP: reference regclass with "pg_catalog." prefix

Strictly speaking this isn't a bug, but since all references to catalog
objects are schema-qualified, we might as well be consistent.  The
omission first appeared in commit 1c5d9270e339, so backpatch to 12.

Author: Justin Pryzby <pryzbyj@telsasoft.com>
Discussion: https://postgr.es/m/20210827193151.GN26465@telsasoft.com

src/bin/psql/describe.c

@@ -4275,12 +4275,12 @@ listPartitionedTables(const char *reltypes, const char *pattern, bool verbose)
 
 	if (showNested || pattern)
 		appendPQExpBuffer(&buf,
-						  ",\n  inh.inhparent::regclass as \"%s\"",
+						  ",\n  inh.inhparent::pg_catalog.regclass as \"%s\"",
 						  gettext_noop("Parent name"));
 
 	if (showIndexes)
 		appendPQExpBuffer(&buf,
-						  ",\n c2.oid::regclass as \"%s\"",
+						  ",\n c2.oid::pg_catalog.regclass as \"%s\"",
 						  gettext_noop("Table"));
 
 	if (verbose)