MXS-1328 Mark regexp functions as builtin functions

Regexp functions were not listed among the builtin read-only
functions and consequently any SELECT using one of those was
routed to master.

query_classifier/test/maxscale.test

@@ -77,3 +77,5 @@ ROLLBACK TO SAVEPOINT id;
 ROLLBACK WORK TO id;
 ROLLBACK TO id;
 RELEASE SAVEPOINT id;
+
+SELECT her FROM (SELECT @@server_id as her) as t WHERE her REGEXP '.*';