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