perf stat: Move perf_stat_synthesize_config() to event.h

Together with the other synthsizers, and rename it to
perf_event__synthesize_stat_events().

This allows us to stop including event.h in util/stat.h.

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-q5ebhrp44txboobs86htu5r9@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>