ptp: ptp_clockmatrix: Fix build error

When do randbuilding, we got this warning:

WARNING: unmet direct dependencies detected for PTP_1588_CLOCK
  Depends on [n]: NET [=y] && POSIX_TIMERS [=n]
  Selected by [y]:
  - PTP_1588_CLOCK_IDTCM [=y]

Make PTP_1588_CLOCK_IDTCM depends on PTP_1588_CLOCK to fix this.

Fixes: 3a6ba7dc ("ptp: Add a ptp clock driver for IDT ClockMatrix.")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Vincent Cheng <vincent.cheng.xh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>