qmake5_base.bbclass: Adding a check to make sure the staging isn't trying to copy into itself

Basically, if you create a reciepe that builds a Qt5 native application, the build will break when trying to execute the do_install() task. This is because the  variable is empty, thus the install is trying to copy files from itself, into itself.
Merge branch 'dora' of https://github.com/Donny3000/meta-qt5 into dora