tools lib traceevent: Add destructor for format_field

Move the calls that frees the resources allocated for a struct format_field to
a separate routine.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1426790181-19118-1-git-send-email-dsahern@gmail.com
[ Split this part from a larger patch, added pevent_ prefix as requested by Steven ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>