Add -lboost_chrono to both the qt .pro file and makefile.unix

On FreeBSD, I was unable to build any version since the MicroSleep() change (commit 1b43bf0d), which created a dependency on boost_chrono for boost versions above 1.50. This dependency had never been added to either makefile.unix or bitcoin-qt.pro.

This pullreq adds the required -lboost_chrono to both files.