AutoTarget ignores Building actors.

Closes #6170 and addresses:
```
[23:22:22]	<pchote> units should only auto-target units
[23:22:27]	<Phrohdoh> why?
[23:22:35]	<pchote> that is how the original games worked 
```

If there is a better way to check caller type let me know and I'll implement it.