perf: Export struct perf_branch_entry to userspace

perf: Export struct perf_branch_entry to userspace

If PERF_SAMPLE_BRANCH_STACK is enabled then samples are returned
with the format { u64 from, to, flags } but the flags layout
is not specified.

This field has the type struct perf_branch_entry; move this
definition into include/uapi/linux/perf_event.h so users can
access these fields.

This is similar to the existing inclusion of perf_mem_data_src in
the include/uapi/linux/perf_event.h file.

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
Acked-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/alpine.DEB.2.10.1308231544420.1889@vincent-weaver-1.um.maine.edu
Signed-off-by: Ingo Molnar <mingo@kernel.org>