Instead rely on struct ethhdr from net/ethernet.h

This fixes build error with musl libc:
In file included from /usr/include/linux/if_bridge.h:17:0,
                 from network.c:47:
/usr/include/linux/if_ether.h:133:8: error: redefinition of 'struct ethhdr'
 struct ethhdr {
        ^
In file included from /usr/include/net/ethernet.h:10:0,
                 from network.c:42:
/usr/include/netinet/if_ether.h:93:8: note: originally defined here
 struct ethhdr {
        ^
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>