perf data: Fix building in 32 bit platform with libbabeltrace

perf data: Fix building in 32 bit platform with libbabeltrace

On ARM32 building it report following error when we build with
libbabeltrace:

  util/data-convert-bt.c: In function 'add_bpf_output_values':
  util/data-convert-bt.c:440:3: error: format '%lu' expects argument of type 'long unsigned int', but argument 5 has type 'unsigned int' [-Werror=format]
  cc1: all warnings being treated as errors

Fix it by changing %lu to %zu.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Fixes: 6122d57e9f7c ("perf data: Support converting data from bpf_perf_event_output()")
Link: http://lkml.kernel.org/r/1475035126-146587-1-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>