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>

tools/perf/tests/task-exit.c

@@ -130,7 +130,7 @@ retry:
 
 out_init:
 	if (!exited || !nr_exit) {
-		perf_evlist__poll(evlist, -1);
+		evlist__poll(evlist, -1);
 		goto retry;
 	}