Fix memory leak in capable.c

This commit adds a cleanup step to free the perf_buffer, fixing a memory leak in the
capable.c file.

libbpf-tools/capable.c

@@ -404,6 +404,7 @@ int main(int argc, char **argv)
  }
 
 cleanup:
+ perf_buffer__free(pb);
  capable_bpf__destroy(obj);
  syms_cache__free(syms_cache);
  ksyms__free(ksyms);