perf stat: Add missing counts.h

It is getting this via evsel.h, that don't strictly need counts.h, just
forward declarations for some structs, so add it here before we remove
it from there.

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-jwcbm9gv9llloe3he5qkdefs@git.kernel.org


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