Only install build event log dlls if the manifest exists under ext

This will allow chefdk to build chef 11

cc @sersut @danielsdeleo 
This should work also:

```ruby
block "Build Event Log Dll" do
  command "#{rake} -rdevkit build_eventlog", cwd: software.project_dir if File.exists? "#{software.project_dir}/ext/win32-eventlog"
end
```