libperf: Move perf's cpu_map__empty() to perf_cpu_map__empty()

libperf: Move perf's cpu_map__empty() to perf_cpu_map__empty()

So it's part of the libperf library as one of basic functions operating
on the perf_cpu_map class.

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 <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190822111141.25823-4-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>