Fix cmake error once the po dir contains translations.

CMake Error at cmake/KF5I18NMacros.cmake:101 (add_custom_target):

The target name "pmapfiles-sr@ijekavian" is reserved or not valid for
certain CMake features, such as generator expressions, and may result in
undefined behavior.

Call Stack (most recent call first):

cmake/KF5I18NMacros.cmake:152 (ki18n_install_ts_files)
CMakeLists.txt:57 (ki18n_install)

We should always test such things with a language that contains '@' in the name :-)

CCMAIL: caslav.ilic@gmx.net