6lowpan: add a license to 6lowpan_iphc module

Since commit 8df8c56a5a, 6lowpan_iphc is a module of its own.

Unfortunately, it lacks some infrastructure to behave like a
good kernel citizen:

  kernel: 6lowpan_iphc: module license 'unspecified' taints kernel.
  kernel: Disabling lock debugging due to kernel taint

This patch adds the basic MODULE_LICENSE(); with GPL license:
the code was copied from net/ieee802154/6lowpan.c which is GPL
and the module exports symbol with EXPORT_SYMBOL_GPL();.

Cc: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Cc: Alexander Aring <alex.aring@gmail.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan_iphc.c

@@ -52,6 +52,7 @@
 
 #include <linux/bitops.h>
 #include <linux/if_arp.h>
+#include <linux/module.h>
 #include <linux/netdevice.h>
 #include <net/ipv6.h>
 #include <net/af_ieee802154.h>
@@ -797,3 +798,5 @@ int lowpan_header_compress(struct sk_buff *skb, struct net_device *dev,
 	return 0;
 }
 EXPORT_SYMBOL_GPL(lowpan_header_compress);
+
+MODULE_LICENSE("GPL");