qa/valgrind.supp: suppress the get-or-create CacheItemHelper

qa/valgrind.supp: suppress the get-or-create CacheItemHelper

```
// Get an CacheItemHelper pointer for value type T and role R.
template <typename T, CacheEntryRole R>
Cache::CacheItemHelper* GetCacheItemHelperForRole() {
  static Cache::CacheItemHelper cache_helper(
      BlocklikeTraits<T>::SizeCallback, BlocklikeTraits<T>::SaveToCallback,
      GetCacheEntryDeleterForRole<T, R>());
  return &cache_helper;
}
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>