perf tools: Move branch structs to branch.h

We already have it, move those there from events.h so that we untangle
the header dependencies a bit more.

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-pnbkqo8jxbi49d4f3yd3b5w3@git.kernel.org


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