add if_ether.h since it not in if.h on some systems.

(Logical change 1.52)
diff --git a/misc/arpd.c b/misc/arpd.c
index 64b563b..7858ab0 100644
--- a/misc/arpd.c
+++ b/misc/arpd.c
@@ -27,6 +27,7 @@
 #include <time.h>
 #include <signal.h>
 #include <linux/if.h>
+#include <linux/if_ether.h>
 #include <linux/if_arp.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>