selftests: timers: Remove local NSEC_PER_SEC and USEC_PER_SEC defines

Remove local NSEC_PER_SEC and USEC_PER_SEC defines. Pick them up from
include/vdso/time64.h. This requires -I $(top_srcdir) to the timers
Makefile to include the include/vdso/time64.h.

posix_timers test names the defines NSECS_PER_SEC and USECS_PER_SEC.
Change posix_timers test references to the defines to match the
defines in the header file.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: John Stultz <jstultz@google.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>