pch_gbe: 'select' NET_PTP_CLASSIFY.

Fixes the following randconfig build failure:

> drivers/net/ethernet/oki-semi/pch_gbe/pch_gbe_main.c: In function
> ‘pch_ptp_match’:
> drivers/net/ethernet/oki-semi/pch_gbe/pch_gbe_main.c:130:2: error:
> implicit declaration of function ‘ptp_classify_raw’
> [-Werror=implicit-function-declaration]
>   if (ptp_classify_raw(skb) == PTP_CLASS_NONE)
>   ^
> cc1: some warnings being treated as errors
> make[5]: *** [drivers/net/ethernet/oki-semi/pch_gbe/pch_gbe_main.o] Error 1

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>