perf utils: Move is_directory() to path.h

perf utils: Move is_directory() to path.h

So that it can be used more widely, like in the next patch, when it will
be used to fix a bug in 'perf test' handling of dirent.d_type ==
DT_UNKNOWN.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20171206174535.25380-1-jolsa@kernel.org
[ Split from a larger patch, removed needless includes in path.h ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>