fixed sampler bank file dialog export

https://bugs.launchpad.net/mixxx/+bug/1403223

Although i did not find the dialog in question, i realized it is the same issue with QFileDialog under Linux as https://github.com/mixxxdj/mixxx/pull/430

I am unsure whether bug occurs under same environment in void SamplerBank::slotLoadSamplerBank() as well. Might need further attention.