perf evsel: Add missing perf/evsel.h header in util/evsel.h

perf evsel: Add missing perf/evsel.h header in util/evsel.h

Since util/evsel.h uses perf_evsel__cpus() that has its prototype in
libperf's perf/evsel.h file, we need it explicitely included.

This was working by luck as util/evsel.h includes counts.h, but that is
not necessary, just some forward declarations, so, before we remove
counts.h from util/evsel.h, add what is realli needed.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-nfb9e0t4jm9zhvr0q86hc29d@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>