ptp: In the testptp utility, use clock_adjtime from glibc when available

clock_adjtime was included in glibc 2.14. _GNU_SOURCE must be defined
to make it available.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Richard Cochran <richardcochran@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>