blktrace: Add blktrace.c to BLOCK LAYER in MAINTAINERS file

blktrace.c is currently maintained by Jens Axboe as it is used for
debugging the block layer. That file should be added to the MAINTAINERS
file under BLOCK LAYER, such that people will know to Cc the BLOCK
LAYER maintainer when modifying that file.

Currently:

 $ scripts/get_maintainer.pl -f kernel/trace/blktrace.c
 Steven Rostedt <rostedt@goodmis.org> (maintainer:TRACING)
 Ingo Molnar <mingo@redhat.com> (maintainer:TRACING)
 linux-kernel@vger.kernel.org (open list)

After the patch:

 $ scripts/get_maintainer.pl -f kernel/trace/blktrace.c
 Jens Axboe <axboe@kernel.dk> (maintainer:BLOCK LAYER)
 Steven Rostedt <rostedt@goodmis.org> (maintainer:TRACING)
 Ingo Molnar <mingo@redhat.com> (maintainer:TRACING)
 linux-kernel@vger.kernel.org (open list)

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jens Axboe <axboe@fb.com>