tools lib traceevent: handle the '->' operator

perf test emits the following warning:

  $ perf test
  5: parse events tests
    ...
    Warning: unknown op '->'
    ...

Add the operator to the checks.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>