[CMake] Bugfix for with SourceGroups Subfolders

[Brad King]

Christopher Lux wrote:
> i ran into a bug with sourcegroups with subfolders/groups. if the parent
> group has no source files the while group with all children was not added to
> the project file. the fix is as follows:

Please create a bug entry here:

http://www.cmake.org/Bug

Once the bug is created you can go back in and attach your patch to it. 
  This way the change will be documented and you will be notified 
automatically when the patch is applied.

Thanks,
-Brad