MAINTAINERS: Add Martin Schiller as a maintainer for the X.25 stack

MAINTAINERS: Add Martin Schiller as a maintainer for the X.25 stack

Martin Schiller is an active developer and reviewer for the X.25 code.
His company is providing products based on the Linux X.25 stack.
So he is a good candidate for maintainers of the X.25 code.

The original maintainer of the X.25 network layer (Andrew Hendry) has
not sent any email to the netdev mail list since 2013. So he is probably
inactive now.

Cc: Andrew Hendry <andrew.hendry@gmail.com>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Acked-by: Martin Schiller <ms@dev.tdt.de>
Link: https://lore.kernel.org/r/20201114111029.326972-1-xie.he.0141@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>