perf test event_update: Fix memory leak of unit

ASan reports a memory leak while running:

  # perf test "49: Synthesize attr update"

Caused by a string being duplicated but never freed.

This patch adds the missing free().

Note that evsel->unit is not deallocated together with evsel since it is
supposed to be a constant string.

Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
Fixes: a6e52817

 ("perf tools: Add event_update event unit type")
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/1fbc8158663fb0d4d5392e36bae564f6ad60be3c.1626343282.git.rickyman7@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>