MAINTAINERS: assign ./fs/tracefs to TRACING

A check with ./scripts/get_maintainer.pl --letters -f fs/tracefs/ shows
that the tracefs is not assigned to the TRACING section in MAINTAINERS.

Add the file pattern for the TRACING section to rectify that.

Link: https://lkml.kernel.org/r/20201109122250.31915-1-lukas.bulwahn@gmail.com



Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>