MAINTAINERS: add 6lowpan header file

Since commit 68d96dcf ("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>