Squish: Add workaround for native file dialogs on Win

Current Squish fails to hook into native file dialogs for
Qt5 builds. The workaround forces Qt Creator to use QFileDialog.

Change-Id: I3a28314bab54178d93cda85c0a36e9202f037c9c
Reviewed-by: Robert Loehning <robert.loehning@digia.com>