libperf: Add counting example

Current libperf man pages mention file counting.c "coming with libperf package",
however, the file is missing. Add the file then.

Fixes: 81de3bf37a8b ("libperf: Add man pages")
Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LPU-Reference: 20200227194424.28210-1-mpetlan@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>