MAINTAINERS: nilfs2: add header file for tracing

This adds header file "include/trace/events/nilfs2.h" to maintainer-ship
of nilfs2 so that updates to the nilfs2 header file go to the mailing list
of nilfs2.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>