networking: fm10k: Fix build failure

The latest linus git tip (3.18-rc1) fails with the following build failure. Fix
this by making PTP support explicit for fm10k driver.

rivers/built-in.o: In function `fm10k_ptp_register':
(.text+0x12e760): undefined reference to `ptp_clock_registER'
drivers/built-in.o: In function `fm10k_ptp_unregister':
(.text+0x12e7dc): undefined reference to `ptp_clock_unregister'
Makefile:930: recipe for target 'vmlinux' failed

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>