MAINTAINERS: Add framer headers to NETWORKING [GENERAL]

The cited commit [1] added framer support under drivers/net/wan,
which is covered by NETWORKING [GENERAL]. And it is implied
that framer-provider.h and framer.h, which were also added
buy the same patch, are also maintained as part of NETWORKING [GENERAL].

Make this explicit by adding these files to the corresponding
section in MAINTAINERS.

[1] 82c944d0 ("net: wan: Add framer framework support")

Signed-off-by: Simon Horman <horms@kernel.org>
Reviewed-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>