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>

MAINTAINERS

@@ -154,6 +154,7 @@ L:	linux-zigbee-devel@lists.sourceforge.net (moderated for non-subscribers)
 L:	linux-bluetooth@vger.kernel.org
 S:	Maintained
 F:	net/6lowpan/
+F:	include/net/6lowpan.h
 
 6PACK NETWORK DRIVER FOR AX.25
 M:	Andreas Koensgen <ajk@comnets.uni-bremen.de>