MAINTAINERS: Add ethtool.h to NETWORKING [GENERAL]

This is part of an effort to assign a section in MAINTAINERS to header
files related to Networking. In this case the files named ethool.h.

Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241210-mnt-ethtool-h-v1-1-2a40b567939d@kernel.org


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