perf session: Fix DEBUG=1 build with clang

perf session: Fix DEBUG=1 build with clang

The struct branch_stack->branch_stack.cycles field is a u64 :16
bitfield, and this somehow confuses clang 4.0 when checking the
arguments of a printf format, so cast the :16 to unsigned short to help
it.

Silences this:

  util/session.c:935:4: error: format specifies type 'unsigned short' but the argument has type 'u64' (aka 'unsigned long') [-Werror,-Wformat]
                          e->flags.cycles,
                          ^~~~~~~~~~~~~~~
  1 error generated.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-eo2t4uhlbne105z72tvyzkp1@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>