[CMake] CMake + Qt5LinguistToolsMacros.cmake: QT5_CREATE_TRANSLATION delete source ts files during clean

[Anatoly Shirokov]

Guys!

I have fixed bug related to delete translation (ts) file during clean 
(someone meet the same problem here 
http://cmake.3232098.n2.nabble.com/How-to-not-delete-generated-files-on-make-clean-td4425991.html) 
for Qt5 (https://bugreports.qt-project.org/browse/QTBUG-31860). But my 
bug report is marked as invalid because they want to have the same 
behaviour for Qt4.

If you can, review my patch and include it into new release for Qt4 
support module.

Thank you very much for your time.

Best regards,
Anatoly Shirokov.