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.