[BOLT][test] Fix NFC check with pre-aggregated-perf.test (#113944)
NFC checks have been failing starting with
https://lab.llvm.org/buildbot/#/builders/92/builds/8567.
NFC testing wrapper (llvm-bolt-wrapper) replaces the call of `perf2bolt`
with `llvm-bolt --aggregate-only --ignore-build-id`.
`show-density` is automatically enabled for perf2bolt only but not for
`llvm-bolt --aggregate-only`. Add the flag to the test to work around
the issue.
Test Plan:
```
cd build
../llvm-project/bolt/utils/nfc-check-setup.py --switch-back --verbose
bin/llvm-lit -a tools/bolt/test/X86/pre-aggregated-perf.test
```