UDF/UDA samples should explicitly export entry points

It's possible for UDF authors to get into a pickle by exporting symbols from UDF/UDA shared objects unintentionally. E.g. if using boost headers, which result in weak symbols in the .so, which then get merged with the (incompatible) symbols in the impalad binary.

We should really be keeping symbols hidden by default then exporting the entry points explicitly in the UDF/UDA code