"perf_serializer.cc:234] Unknown event type: 19" with latest Linux tree

Running create_llvm_prof on a profile generated on Linux 5.4.0:
```
perf record -o cycles.perf.data -e cycles -- ...
create_llvm_prof -format=text -binary=./hotcold -profile=cycles.perf.data -out=cycles.llvm.prof
```
fails with
```
E0709 15:46:56.726442 4116 perf_serializer.cc:234] Unknown event type: 19
E0709 15:46:56.726524 4116 perf_serializer.cc:234] Unknown event type: 19
E0709 15:46:56.728390 4116 perf_parser.cc:248] Unknown event type: 19
```
This is PERF_RECORD_CGROUP, which is now generated by the kernel. It shouldn't be a problem to ignore it when consuming a perf.data file. More generally, all create_llvm_prof needs to do is consume the perf data - couldn't it warn about any unknown types and continue?