MAINTAINERS: Add in6.h to MAINTAINERS

My CC-adding automation returned nothing on a future patch to the
include/linux/in6.h file, and I went looking for why. Add the missed
in6.h to MAINTAINERS.

Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250722165645.work.047-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kees

MAINTAINERS

@@ -17384,6 +17384,7 @@ F:	include/linux/ethtool.h
 F:	include/linux/framer/framer-provider.h
 F:	include/linux/framer/framer.h
 F:	include/linux/in.h
+F:	include/linux/in6.h
 F:	include/linux/indirect_call_wrapper.h
 F:	include/linux/inet.h
 F:	include/linux/inet_diag.h