configure.ac: fix <netinet/if_ether.h> detection on OpenBSD

[Austin English]

<netinet/if_ether.h> depends on <net/if.h> to define ac_if and
<net/if_arp.h> to define ea_hdr on OpenBSD

-- 
-Austin
-------------- next part --------------
diff --git a/configure.ac b/configure.ac
index a420de2..271a370 100644
--- a/configure.ac
+++ b/configure.ac
@@ -399,6 +399,12 @@ AC_CHECK_HEADERS(\
      #ifdef HAVE_SYS_SOCKETVAR_H
      # include <sys/socketvar.h>
      #endif
+     #ifdef HAVE_NET_IF_H
+     # include <net/if.h>
+     #endif
+     #ifdef HAVE_NET_IF_ARP_H
+     # include <net/if_arp.h>
+     #endif
      #ifdef HAVE_NETINET_IN_H
      # include <netinet/in.h>
      #endif