util/u_thread: Restrict u_thread_get_time_nano on macOS.

util/u_thread: Restrict u_thread_get_time_nano on macOS.

macOS does not have pthread_getcpuclockid.

src/util/u_thread.h:156:4: error: implicit declaration of function 'pthread_getcpuclockid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
   pthread_getcpuclockid(thread, &cid);
   ^

Fixes: 4913215d145e ("util/u_thread: don't restrict u_thread_get_time_nano() to __linux__")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2171
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Eric Engestrom <eric@engestrom.ch>