Restore erroneously removed ONLY from PK check

This is a blind fix, since I don't have SE-Linux to verify it.

Per unwanted change in rhinoceros, running sepgsql tests.  Noted by Tom
Lane.

Discussion: https://postgr.es/m/32347.1522865050@sss.pgh.pa.us

src/backend/utils/adt/ri_triggers.c

@@ -1915,7 +1915,7 @@ RI_Initial_Check(Trigger *trigger, Relation fk_rel, Relation pk_rel)
 	/*----------
 	 * The query string built is:
 	 *	SELECT fk.keycols FROM [ONLY] relname fk
-	 *	 LEFT OUTER JOIN pkrelname pk
+	 *	 LEFT OUTER JOIN ONLY pkrelname pk
 	 *	 ON (pk.pkkeycol1=fk.keycol1 [AND ...])
 	 *	 WHERE pk.pkkeycol1 IS NULL AND
 	 * For MATCH SIMPLE:
@@ -1943,7 +1943,7 @@ RI_Initial_Check(Trigger *trigger, Relation fk_rel, Relation pk_rel)
 	fk_only = fk_rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE ?
 		"" : "ONLY ";
 	appendStringInfo(&querybuf,
-					 " FROM %s%s fk LEFT OUTER JOIN %s pk ON",
+					 " FROM %s%s fk LEFT OUTER JOIN ONLY %s pk ON",
 					 fk_only, fkrelname, pkrelname);
 
 	strcpy(pkattname, "pk.");