Fix another test for RELKIND_RELATION that should allow foreign tables now.

I thought I'd gone through all of these before, but a fresh review found
this one too.  (Perhaps it would be better to just delete this test and
let the failure occur later, but for the moment I'll preserve the logic.)

The case that this was rejecting is like
	CREATE FOREIGN TABLE ft (f1 int ...) ...;
	CREATE TABLE c1 (UNIQUE(f1)) INHERITS(ft);

src/backend/parser/parse_utilcmd.c

@@ -1799,10 +1799,12 @@ transformIndexConstraint(Constraint *constraint, CreateStmtContext *cxt)
 
 				Assert(IsA(inh, RangeVar));
 				rel = heap_openrv(inh, AccessShareLock);
-				if (rel->rd_rel->relkind != RELKIND_RELATION)
+				/* check user requested inheritance from valid relkind */
+				if (rel->rd_rel->relkind != RELKIND_RELATION &&
+					rel->rd_rel->relkind != RELKIND_FOREIGN_TABLE)
 					ereport(ERROR,
 							(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-						   errmsg("inherited relation \"%s\" is not a table",
+						   errmsg("inherited relation \"%s\" is not a table or foreign table",
 								  inh->relname)));
 				for (count = 0; count < rel->rd_att->natts; count++)
 				{