Xcode: Fix duplicate target subfolders (#14133)

Fix logic introduced by commit eeeeca10 (XCode: Support target folders
on XCode, 2011-02-20) to avoid duplicate subfolders.  The problem was
that no slash was appended to the curr_tgt_folder string on the it !=
this->TargetGroup.end() path.
Merge topic 'xcode-folder-dedup' into next

2dcb1dc Xcode: Fix duplicate target subfolders (#14133)