[Cmake] Apparent bug in the SOURCE_GROUP command, and a fix

[Brad King]

David,

>    for ( cc = 3 ; cc < args.size() ; cc ++ )
>
> Apparently someone forgot (in just one place) that the first
> element of args is args[0], not args[1].  The bug appears to be
> fixed when I changed the 3 to a 2 in the line of code above.

I've committed the fix to the CVS version.  Since this doesn't affect the
success of a build, we've never noticed this before.

Thanks,
-Brad