Build: support strip-po target for translations
    
    The target was added to top-level Makefile.am as well so the maintainer
    does not need to jump between directories when doing Zanata pull/push
    and strip-po.
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>