ucalls: convert bytes to str

This fixes the following error with python3

Tracing calls in process 2577 (language: python)... Ctrl-C to quit.
^C
Traceback (most recent call last):
  File "/usr/share/bcc/tools/lib/ucalls", line 303, in <module>
    data = get_data()   # [(function, (num calls, latency in ns))]
  File "/usr/share/bcc/tools/lib/ucalls", line 264, in get_data
    bpf["counts"].items()))
  File "/usr/share/bcc/tools/lib/ucalls", line 262, in <lambda>
    data = list(map(lambda kv: (kv[0].clazz + "." + kv[0].method,
TypeError: can't concat str to bytes

Signed-off-by: Rafael Fonseca <r4f4rfs@gmail.com>