perf event: Move perf_event__synthesize* to event.h

Where is the perf_event__handler_t typedef they need, which was the only
reason for header.h to be including event.h, untangle that.

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-outjyzh1o29ndcv9lsqyzt87@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>