[2893] BPF header includes common header for ethernet constants.

The if_ether.h is portable across various BSD system flavors. The
ethernet.h is missing on some OSes.

src/lib/dhcp/pkt_filter_bpf.cc

@@ -21,7 +21,7 @@
 #include <exceptions/exceptions.h>
 #include <algorithm>
 #include <net/bpf.h>
-#include <net/ethernet.h>
+#include <netinet/if_ether.h>
 
 namespace {