Put back allow_system_table_mods check in heap_create().

This reverts commit a475c60367.

Erik Rijkers reported back in January 2013 that after the patch, if you do
"pg_dump -t myschema.mytable" to dump a single table, and restore that in
a database where myschema does not exist, the table is silently created in
pg_catalog instead. That is because pg_dump uses
"SET search_path=myschema, pg_catalog" to set schema the table is created
in. While allow_system_table_mods is not a very elegant solution to this,
we can't leave it as it is, so for now, revert it back to the way it was
previously.

src/backend/catalog/index.c

@@ -825,7 +825,8 @@ index_create(Relation heapRelation,
 								RELKIND_INDEX,
 								relpersistence,
 								shared_relation,
-								mapped_relation);
+								mapped_relation,
+								allow_system_table_mods);
 
 	Assert(indexRelationId == RelationGetRelid(indexRelation));