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_iphc.c b/net/ieee802154/6lowpan_iphc.c
index 860aa2d..211b568 100644
--- a/net/ieee802154/6lowpan_iphc.c
+++ b/net/ieee802154/6lowpan_iphc.c
@@ -54,11 +54,10 @@
 #include <linux/if_arp.h>
 #include <linux/module.h>
 #include <linux/netdevice.h>
+#include <net/6lowpan.h>
 #include <net/ipv6.h>
 #include <net/af_ieee802154.h>
 
-#include "6lowpan.h"
-
 /*
  * Uncompress address function for source and
  * destination address(non-multicast).