perf trace: Free strings in trace__parse_events_option()

ASan reports several memory leaks running:

  # perf test "88: Check open filename arg using perf trace + vfs_getname"

The fourth of these leaks is related to some strings never being freed
in trace__parse_events_option.

This patch adds the missing frees.

Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/34d08535b11124106b859790549991abff5a7de8.1626343282.git.rickyman7@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>