Fix building on OpenBSD

[brad0]

In file included from ../../src/common/sendpacket.h:30: 
/usr/include/netinet/if_ether.h:154:17: fatal error: field has incomplete type 'struct arphdr'
        struct   arphdr ea_hdr;                 /* fixed-size header */
                        ^
/usr/include/netinet/if_ether.h:154:10: note: forward declaration of 'struct arphdr'
        struct   arphdr ea_hdr;                 /* fixed-size header */
                 ^
1 error generated.

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