tools lib api: Unexport 'tracing_path' variable

One should use tracing_path_mount() instead, so more things get done
lazily instead of at every 'perf' tool call startup.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-fci4yll35idd9yuslp67vqc2@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>