Make bool -> int conversion explicit to silence VC build warnings.

```
(ClCompile target) ->
  template.c(6650): warning C4805: '|=' : unsafe mix of type 'int' and type 'bool' in operation [
C:\dmd-git\dmd2\src\dmd\src\dmd_ms
```

It's a pretty "meh" change, and is probably safe in D.