Made test t/87async.t not stop on Win32.

This test used Time::HiRes::clock_gettime() but this gives a compile
time warning on MSWin32.
I replaced this with Time::HiRes::gettimeofday() so this test no
longer fails there.
Merge pull request #6 from mbeijen/fix-87-async-win32

Made test t/87async.t not stop on Win32.