MAINTAINERS: add mec as second maintainer for gdb.threads

[Michael Chastain]

Andrew and Michael Snyder and I have talked about adding me as
a second maintainer for the gdb.threads tests:

  threads (gdb.threads) Michael Snyder          msnyder@redhat.com
 +                      Michael Chastain        mec.gnu@mindspring.com

The conversation petered out with approval, but no action.

How about it?

Who is the maintainer of MAINTAINERS ... do I go to the steering
committee and ask for a vote?

Michael