MAINTAINERS: add Dan Streetman to zswap maintainers

Add myself as zswap maintainer.

Link: http://lkml.kernel.org/r/20170124212200.19052-1-ddstreet@ieee.org
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Acked-by: Seth Jennings <sjenning@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>