mysqld_slower: Fix breakage after USDT API change

A recent PR to the USDT infrastructure changed the BPF
module constructor to accept an array of USDT contexts.
This commit updates `mysqld_slower` to use that new
constructor.

tools/mysqld_qslower.py

@@ -102,7 +102,7 @@ def usage():
  print(bpf_text)
 
 # initialize BPF
-b = BPF(text=bpf_text, usdt=u)
+b = BPF(text=bpf_text, usdt_contexts=[u])
 
 # header
 print("Tracing MySQL server queries for PID %d slower than %s ms..." % (pid,