net: ptp: Add stub for ptp_classify_raw()

When NET_PTP_CLASSIFY is disabled, a stub function is required in
order that the drivers compile.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ptp_classify.h

@@ -75,5 +75,9 @@ void __init ptp_classifier_init(void);
 static inline void ptp_classifier_init(void)
 {
 }
+static inline unsigned int ptp_classify_raw(struct sk_buff *skb)
+{
+	return PTP_CLASS_NONE;
+}
 #endif
 #endif /* _PTP_CLASSIFY_H_ */