6lowpan: add missing include of net/ipv6.h

The 6lowpan.h file contains some static inline function which use
internal ipv6 api structs. Add a include of ipv6.h to be sure that it's
known before.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan.h

@@ -53,6 +53,8 @@
 #ifndef __6LOWPAN_H__
 #define __6LOWPAN_H__
 
+#include <net/ipv6.h>
+
 #define UIP_802154_SHORTADDR_LEN	2  /* compressed ipv6 address length */
 #define UIP_IPH_LEN			40 /* ipv6 fixed header size */
 #define UIP_PROTO_UDP			17 /* ipv6 next header value for UDP */