Avoid compiler warning in non-assert builds

After 01575ad788, expand_single_inheritance_child()'s parentOID
variable is read only in an Assert, provoking a compiler warning in
non-assert builds.  Fix that by marking the variable with
PG_USED_FOR_ASSERTS_ONLY.

Per report and suggestion from David Rowley

Discussion: https://postgr.es/m/CAApHDvpjA_8Wxu4DCTRVAvPxC9atwMe6N%2ByvrcGsgb7mrfdpJA%40mail.gmail.com

src/backend/optimizer/util/inherit.c

@@ -457,7 +457,8 @@ expand_single_inheritance_child(PlannerInfo *root, RangeTblEntry *parentrte,
 								Index *childRTindex_p)
 {
 	Query	   *parse = root->parse;
-	Oid			parentOID = RelationGetRelid(parentrel);
+	Oid			parentOID PG_USED_FOR_ASSERTS_ONLY =
+		RelationGetRelid(parentrel);
 	Oid			childOID = RelationGetRelid(childrel);
 	RangeTblEntry *childrte;
 	Index		childRTindex;