Export my_progname symbol in libmysqlclient.so

This issue is related to MDEV-5529. We started to use a different patch in Red Hat before mariadb source started to export only some symbols in libmysqlclient.so. We have also exported 'my_progname' symbol.

As we're rebasing to 5.5.37 now, we do not use our own (downstream) symbol versioning any more, but 'my_progname' symbol is missing (which is not acceptable for us, since it would introduce ABI incompatibility).

Please, export that symbol too, so we really have compatible client libraries.

Thanks.