mac-capture: Adjust mHostTime to milliseconds

The timestamp returned by mHostTime in the AudioTimeStamp structure
is the current timestamp equivalent to mach_absolute_time(), which
is relative to the machine's time base. In order to convert this
to milliseconds, it's necessary to get the host's timebase with
mach_timebase_info() and scale the timestamp accordingly, since
the rest of the timestamp synchronization code expects the timestamp
to be in milliseconds.

This is effectively equivalent to the code which was previously in
libobs/util/platform-coca.m, but must be applied here instead.