Only add the res_file to the generated files if there is no rc_file

If both the rc_file and res_file was added to the project then in MSVC
2012 it would cause an error complaining of a duplicated resource.

Task-number: QTBUG-29826
Change-Id: Ib3401b423208cd8c90895038c9cbadd80c39b542
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/fa1fb5ed945650d45cf9b61906232cfbae11360e)
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>