net: Add skbuff.h to MAINTAINERS

The network maintainers need to be copied if the skbuff.h is touched.

This also helps git-send-email to figure out the proper maintainers when
touching the file.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240730161404.2028175-1-leitao@debian.org


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