perf test maps__merge_in: Fix memory leak of maps

ASan reports a memory leak when running:

  # perf test "65: maps__merge_in"

This is the second and final patch addressing these memory leaks.

This time, the problem is simply that the maps object is never
destructed.

This patch adds the missing maps__exit call.

Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
Fixes: 79b6bb73

 ("perf maps: Merge 'struct maps' with 'struct map_groups'")
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/a1a29b97a58738987d150e94d4ebfad0282fb038.1626343282.git.rickyman7@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>