Make sure that the tsfiles target is generated

Summary:
I'm not sure how it's possible that pofiles is defined but tsfiles
isn't, but this does fix the issue reported by Luigi:
CMake Error at /usr/lib64/cmake/KF5I18n/KF5I18NMacros.cmake:129
(add_dependencies):

Cannot add target-level dependencies to non-existent target "tsfiles".

Test Plan:
Built svn+ssh://svn@svn.kde.org/home/kde/trunk/l10n-kf5/fr by doing
./scripts/autogen.sh fr; mkdir build; cd build; cmake ../fr

Reviewers: Frameworks, ltoscano, lueck, aacid

Reviewed By: ltoscano

Subscribers: aacid, asturmlechner

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6331

(cherry picked from commit 77eab970fde3ecbc259d6deff61fce4f360b05a7)