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 <[email protected]>
Signed-off-by: Marcel Holtmann <[email protected]>
diff --git a/MAINTAINERS b/MAINTAINERS
index 0057ff5..1859c2d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -154,6 +154,7 @@
 L:	[email protected]
 S:	Maintained
 F:	net/6lowpan/
+F:	include/net/6lowpan.h
 
 6PACK NETWORK DRIVER FOR AX.25
 M:	Andreas Koensgen <[email protected]>