perf hist: Add missing 'struct branch_stack' forward declaration

Its needed, was being obtained 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-srzphk0ehptfn3zqmpkgsi65@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>