always process po dir if present

this fixes l10n integration magic from ECM (KDE_L10N_SYNC_TRANSLATIONS,
KDE_L10N_AUTO_TRANSLATIONS) that download the po dir into the build dir
for git builds.

ki18n_install is looking for po dirs and not doing anything when it
can't find any. so, by default this changes nothing. when used with the
aforementioned options it enables translation building for git builds.
for tarball builds this has no negative impact since releaseme is smart
enough to check if a cmakelists already contains the ki18n call before
append its own.