libperf: Add perf_evlist__poll() function

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

And rename the existing perf's function to evlist__poll().

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/20190913132355.21634-39-jolsa@kernel.org


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