perf tools: Use asprintf() for simple string formatting/allocation

perf tools: Use asprintf() for simple string formatting/allocation

No need to use strbuf there, its just a simple alloc+formatting, which
asprintf does just fine.

Cc: Adrian Hunter <adrian.hunter@intel.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-6q6cxfhk8c8ypg3tfpo0i2iy@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>