MAINTAINERS: add 6lowpan header file
Since commit 68d96dcfc6c09b565d57897c127b61afbab74c6f ("MAINTAINERS: add
net/6lowpan/ maintainer entry") we have a 6lowpan branch. This patch
adds a forgotten file which should also be maintained by this branch.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>