6lowpan: move 6lowpan header to include/net

This header is used by bluetooth and ieee802154 branch. This patch
move this header to the include/net directory to avoid a use of a
relative path in include.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ieee802154/6lowpan_rtnl.c b/net/ieee802154/6lowpan_rtnl.c
index e472618..1bbab89 100644
--- a/net/ieee802154/6lowpan_rtnl.c
+++ b/net/ieee802154/6lowpan_rtnl.c
@@ -52,10 +52,10 @@
 #include <net/af_ieee802154.h>
 #include <net/ieee802154.h>
 #include <net/ieee802154_netdev.h>
+#include <net/6lowpan.h>
 #include <net/ipv6.h>
 
 #include "reassembly.h"
-#include "6lowpan.h"
 
 static LIST_HEAD(lowpan_devices);