Bluetooth: Add missing 6lowpan.h include

The 6lowpan.c file was missing an #include statement for 6lowpan.h.
Without it we get the following type of warnings:

net/bluetooth/6lowpan.c:320:5: warning: symbol 'bt_6lowpan_recv' was not declared. Should it be static?
net/bluetooth/6lowpan.c:737:5: warning: symbol 'bt_6lowpan_add_conn' was not declared. Should it be static?
net/bluetooth/6lowpan.c:805:5: warning: symbol 'bt_6lowpan_del_conn' was not declared. Should it be static?
net/bluetooth/6lowpan.c:878:5: warning: symbol 'bt_6lowpan_init' was not declared. Should it be static?
net/bluetooth/6lowpan.c:883:6: warning: symbol 'bt_6lowpan_cleanup' was not declared. Should it be static?
Signed-off-by: NJohan Hedberg <johan.hedberg@intel.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/bluetooth/6lowpan.c

@@ -26,6 +26,8 @@
 #include <net/bluetooth/hci_core.h>
 #include <net/bluetooth/l2cap.h>
 
+#include "6lowpan.h"
+
 #include "../ieee802154/6lowpan.h" /* for the compression support */
 
 #define IFACE_NAME_TEMPLATE "bt%d"