translations: Use install(FILES ...) instead of install(DIRECTORY ...)

SCREENGRAB_QMS holds a list of files not directories.
It succeeds because CMake treats each file as being it's directory. But it
fails when for some reason SCREENGRAB_QMS is reevaluated.
Example:
    cmake .. && make && sudo make install
    cmake -DCMAKE_INSTALL_PREFIX=/usr/local ..

The later call to cmake fails.