Use wxGetUTCTimeUSec() in wxStopWatch under Unix for higher precision.

If gettimeofday() is available we can achieve better than millisecond
precision (even if it usually isn't as high as microsecond), so use it as
clock source in wxStopWatch.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69841 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775