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/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c
index adb3ea0..73492b9 100644
--- a/net/bluetooth/6lowpan.c
+++ b/net/bluetooth/6lowpan.c
@@ -27,7 +27,7 @@
 
 #include "6lowpan.h"
 
-#include "../ieee802154/6lowpan.h" /* for the compression support */
+#include <net/6lowpan.h> /* for the compression support */
 
 #define IFACE_NAME_TEMPLATE "bt%d"
 #define EUI64_ADDR_LEN 8