MAINTAINERS: Add sctp headers to the general netdev entry

All SCTP patches are picked up by netdev maintainers. Two headers were
missing to be listed there.

Reported-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/b3c2dc3a102eb89bd155abca2503ebd015f50ee0.1739193671.git.marcelo.leitner@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>