MAINTAINERS: Add net_tstamp.h to SOCKET TIMESTAMPING section

This is part of an effort to assign a section in MAINTAINERS to header
files that relate to Networking. In this case the files with "net" in
their name.

Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Simon Horman <horms@kernel.org>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>