libperf: Add perf_evlist__alloc_pollfd() function

Move perf_evlist__alloc_pollfd() from tools/perf to libperf, it will be
used in the following patches.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lore.kernel.org/lkml/20190913132355.21634-37-jolsa@kernel.org


[ Added api/fd/array.h include to the lib/evlist.c file ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>