6lowpan: add a license to 6lowpan_iphc module

6lowpan: add a license to 6lowpan_iphc module

Since commit d410f8cbe249, 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>