drivers: eth: native_posix: Fix gPTP compile error
drivers: eth: native_posix: Fix gPTP compile error

If compiling with gPTP support for native_posix board, then avoid
compile error that is seen with gptp sample application.

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

nashif/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,ldts/zephyr,punitvara/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,ldts/zephyr