perf callchain: Remove needless event.h include

All we need is a bunch of struct forward declarations and then add
event.h to the only place that was getting it indirectly via
callchain.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-qq2xhyuxcvx5vmxha9otjd8d@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>