0.19.0: offwaketime: filtering by PID does not caputure any stacks

Steps to reproduce:

```
git clone git://github.com/asheplyakov/profilingmulticore.git
cd profilingmulticore
git checkout 20210429
make bin/thunderingherd
sudo /bin/true && ./bin/thunderingherd & pid=$!; echo "tracing $pid"; sudo env PYTHONPATH=/opt/bcc/lib/python3/dist-packages LD_LIBRARY_PATH=/opt/bcc/lib /opt/bcc/share/bcc/tools/offwaketime -f -p $pid 8 > stacks; echo "finished tracing $pid"; wait $pid; wc -l stacks
```

The `stacks` file is empty:

```
[1] 638635
tracing 638635
6 worker threads
producer: message period 10 usec
workers: service time 1 usec
ETA: 10 sec
finished tracing 638635
Actual time: 12541 msec
[1]+ Done sudo /bin/true && ./bin/thunderingherd
0 stacks
```

This is on Ubuntu 20.10 with kernel 5.8, llvm-8-dev, libclang-8-dev