Prefer linux/if_ether.h over netinet/if_ether.h if available since the
musl libc if_ether.h header does not allow mixing with kernel headers,
it will result in a struct ethhdr redefinition error.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>