perf sched: Add missing event.h include directive

We use what is defined there, were getting it by luck, indirectly, fix
it.

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-e1cdt9557ctpvs3jb9c16qe6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>