Fixed twister-qt.pro for boost > 1.46

[edited to make things more clear]
Twister must be linked to libboost_chrono when building on unix with boost > 1.46. I have fixed twister.qt-pro to do so, but I'm not sure (and can't test at the moment), if it still works with boost < 1.46, since the code tests for version 1.46 only.