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 <[e-mail]>
Signed-off-by: Xie He <[e-mail]>
Acked-by: Martin Schiller <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

MAINTAINERS

···
 F:	arch/mips/lantiq
 F:	drivers/soc/lantiq
 
-LAPB module
-L:	[e-mail]
-S:	Orphan
-F:	Documentation/networking/lapb-module.rst
-F:	include/*/lapb.h
-F:	net/lapb/
-
 LASI 53c700 driver for PARISC
 M:	"James E.J. Bottomley" <[e-mail]>
 L:	[e-mail]
···
 S:	Maintained
 N:	axp[128]
 
-X.25 NETWORK LAYER
-M:	Andrew Hendry <[e-mail]>
+X.25 STACK
+M:	Martin Schiller <[e-mail]>
 L:	[e-mail]
-S:	Odd Fixes
+S:	Maintained
+F:	Documentation/networking/lapb-module.rst
 F:	Documentation/networking/x25*
+F:	drivers/net/wan/hdlc_x25.c
+F:	drivers/net/wan/lapbether.c
+F:	include/*/lapb.h
 F:	include/net/x25*
+F:	include/uapi/linux/x25.h
+F:	net/lapb/
 F:	net/x25/
 
 X86 ARCHITECTURE (32-BIT AND 64-BIT)