usdt: fix argument passing on python3 (iovisor#984)

This fixes the following error:

$: ./tplist -v -v -l /usr/lib64/dri/i965_dri.so
argument 1: <class 'TypeError'>: wrong type

src/python/bcc/usdt.py

@@ -130,7 +130,7 @@ def __init__(self, pid=None, path=None):
                 raise USDTException("USDT failed to instrument PID %d" % pid)
         elif path:
             self.path = path
-            self.context = lib.bcc_usdt_new_frompath(path)
+            self.context = lib.bcc_usdt_new_frompath(path.encode('ascii'))
             if self.context == None:
                 raise USDTException("USDT failed to instrument path %s" % path)
         else: