Inclusion of mach/mach_time.h by libcxx/src/chrono.cpp breaks
the build on latest MACOS dues to https://forums.developer.apple.com/forums/thread/746737
The interface from mach/mach_time.h is not referenced in chrono.cpp
so it is safe to remove it
Signed-off-by: Petro Karashchenko <petro.karashchenko@gmail.com>
