ath6kl: include in.h explicitly

Do not assume anyone before us includes it for us.
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath6kl/wmi.c

@@ -16,6 +16,7 @@
  */
 
 #include <linux/ip.h>
+#include <linux/in.h>
 #include "core.h"
 #include "debug.h"
 #include "testmode.h"