kernel: add missing symbol when enabling PTP support

Discovered by enabling PTP_1588_CLOCK:

net/sched/Kconfig:44: warning: menuconfig statement without prompt
*
* Restart config...
*
*
* PTP clock support
*
PTP clock support (PTP_1588_CLOCK) [Y/n/?] y
Driver for the National Semiconductor DP83640 PHYTER (DP83640_PHY) [N/m/y/?] (NEW)

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

target/linux/generic/config-4.14

@@ -1149,6 +1149,7 @@ CONFIG_DEVPORT=y
 # CONFIG_DNOTIFY is not set
 # CONFIG_DNS_RESOLVER is not set
 CONFIG_DOUBLEFAULT=y
+# CONFIG_DP83640_PHY is not set
 # CONFIG_DP83848_PHY is not set
 # CONFIG_DP83867_PHY is not set
 # CONFIG_DPOT_DAC is not set