perf ui browser: Add vprintf() method

perf ui browser: Add vprintf() method

We'll need it for some callbacks for the upcoming
annotation__line_print() routines.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-t3qiobj4ua38xzsq8cyw9ky5@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>