perf stat: Use nftw() instead of ftw()

perf stat: Use nftw() instead of ftw()

ftw() has been obsolete for about 12 years now.

Committer notes:

Further notes provided by the patch author:

    "NOTE: Not runtime-tested, I have no idea what I need to do in perf
     to test this. But at least it compiles now with my uClibc-based
     toolchain."

I looked at the nftw()/ftw() man page and for the use made with cgroups
in 'perf stat' the end result is equivalent.

Fixes: f52676ff132c ("perf stat: Support regex pattern in --for-each-cgroup")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.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>
Cc: od@zcrc.me
Cc: stable@vger.kernel.org
Link: http://lore.kernel.org/lkml/20210208181157.1324550-1-paul@crapouillou.net
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>