perf report: Free generated help strings for sort option

ASan reports the memory leak of the strings allocated by sort_help() when
running perf report.

This patch changes the returned pointer to char* (instead of const
char*), saves it in a temporary variable, and finally deallocates it at
function exit.

Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
Fixes: 702fb9b4

 ("perf report: Show all sort keys in help output")
Cc: Andi Kleen <ak@linux.intel.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/a38b13f02812a8a6759200b9063c6191337f44d4.1626343282.git.rickyman7@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>