perf tools: Move event synthesizing routines to separate header

Those are the only routines using the perf_event__handler_t typedef and
are all related, so move to a separate header to reduce the header
dependency tree, lots of places were getting event.h and even stdio.h,
limits.h indirectly, so fix those as well.

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-yvx9u1mf7baq6cu1abfhbqgs@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -61,6 +61,7 @@
 #include "util/tool.h"
 #include "util/string2.h"
 #include "util/metricgroup.h"
+#include "util/synthetic-events.h"
 #include "util/target.h"
 #include "util/time-utils.h"
 #include "util/top.h"