tools: fix runqslower warning

The state member of task_struct is volatile and it's use as the last
parameter of bpf_probe_read (const void *) triggers the following
clang warning (LLVM 8):

/virtual/main.c:56:42: warning: passing 'volatile long *' to parameter of type 'const void *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
    bpf_probe_read(&state, sizeof(long), &prev->state);
                                         ^~~~~~~~~~~~
1 warning generated.
Tracing run queue latency higher than 10000 us
TIME     COMM             PID           LAT(us)

An explicit cast fixes the warning.

tools/runqslower.py

@@ -167,7 +167,7 @@
  long state;
 
  // ivcsw: treat like an enqueue event and store timestamp
- bpf_probe_read(&state, sizeof(long), &prev->state);
+ bpf_probe_read(&state, sizeof(long), (const void *)&prev->state);
  if (state == TASK_RUNNING) {
  bpf_probe_read(&tgid, sizeof(prev->tgid), &prev->tgid);
  bpf_probe_read(&pid, sizeof(prev->pid), &prev->pid);