mariadb: Easy linkage of code using libmysqlclient_r

The libmysqlclient_r library is now a symbolic link for the
libmysqlclient one, however some code still requires it for
linking.

To easy the transition, when installing the libmysqlclient-dev we
should include the libmysqlclient-r-dev as well.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>