perf tools: Remove include dirent.h from util.h

perf tools: Remove include dirent.h from util.h

The files using the dirent.h routines should instead include it,
reducing the includes hell that lead to longer build times.

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: http://lkml.kernel.org/n/tip-42g2f4z6nfg7mdb2ae97n7tj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>