In python 3, map returns an iterator and not a list anymore. This
patch cast the map into a list. It fixes the following error:

$ /usr/share/bcc/tools/dbstat mysql
Traceback (most recent call last):
  File "/usr/share/bcc/tools/dbstat", line 95, in <module>
    bpf = BPF(text=program, usdt_contexts=usdts)
  File "/usr/lib/python3.6/site-packages/bcc/__init__.py", line 339, in __init__
    ctx_array = (ct.c_void_p * len(usdt_contexts))()
TypeError: object of type 'map' has no len()