Fix compilation with Xcode 12.5+
Fix compilation with Xcode 12.5+

Mark manager.cpp as containing Objective-C++.

Fixes #717

PKRoma/poedit,vslavik/poedit,vslavik/poedit,alexhenrie/poedit,alexhenrie/poedit,nawawi/poedit,PKRoma/poedit,vslavik/poedit,nawawi/poedit,alexhenrie/poedit,PKRoma/poedit,alexhenrie/poedit,vslavik/poedit,nawawi/poedit,alexhenrie/poedit,PKRoma/poedit,nawawi/poedit,PKRoma/poedit,vslavik/poedit,nawawi/poedit