* Fix USDT probes arguments' encoding in Python3

Running `trace` on a binary's USDT while fetching some arguments (
`sudo python3 trace.py -p $(pidof ruby) 'u:ruby:array__create "%d",
arg1'`) fails with `argument 2: <class 'TypeError'>: wrong type`.

This PR fixes the encoding of the USDT probe name in
udst.py `get_probe_arg_ctype` function. I've tested this works on Python 2 too.