perf inject: Fix missing free in copy_kcore_dir()

Free string allocated by asprintf().

Fixes: d8fc0855

 ("perf inject: Keep a copy of kcore_dir")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220620103904.7960-1-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>