perf intel-pt: Use allocated branch stack for PEBS sample

perf intel-pt: Use allocated branch stack for PEBS sample

To avoid having struct branch_stack as a non-last structure member,
use allocated branch stack for PEBS sample.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/2540ed9a-89f1-6d59-10c9-a66cc90db5d2@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>