Avoid core dump in pg_dump when dumping from pre-8.3 server.

Commit f0e21f2f6 missed adding a tgisinternal output column
to getTriggers' query for pre-8.3 servers.  Back-patch to v11,
like that commit.

src/bin/pg_dump/pg_dump.c

@@ -8082,6 +8082,7 @@ getTriggers(Archive *fout, TableInfo tblinfo[], int numTables)
 							  "SELECT tgname, "
 							  "tgfoid::pg_catalog.regproc AS tgfname, "
 							  "tgtype, tgnargs, tgargs, tgenabled, "
+							  "false as tgisinternal, "
 							  "tgisconstraint, tgconstrname, tgdeferrable, "
 							  "tgconstrrelid, tginitdeferred, tableoid, oid, "
 							  "tgconstrrelid::pg_catalog.regclass AS tgconstrrelname "