Fix building on OpenBSD

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.

src/common/sendpacket.h

@@ -27,6 +27,7 @@
 #ifdef __NetBSD__
 #include <net/if_ether.h>
 #elif ! defined(__HAIKU__)
+#include <net/if.h>
 #include <netinet/if_ether.h>
 #endif