perf machine: Fix missing free of machine->kallsyms_filename

perf machine: Fix missing free of machine->kallsyms_filename

Add missing free of machine->kallsyms_filename to machine__exit().

Fixes: a5367ecb5353fbf2 ("perf tools: Automatically use guest kcore_dir if present")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220809130758.12800-1-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>