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>

.gitignore

@@ -5,6 +5,7 @@
 /ABOUT-NLS
 /po/*.gmo
 /po/*~
+/po/Makefile.hack
 /po/Makefile.in.in
 /po/Makevars.template
 /po/POTFILES