gio: fix compiling on macOS by moving all Objective-C file into libgio-objc-2.0
Previously, only gnextstepsettingsbackend.c was taken into account. https://bugzilla.gnome.org/show_bug.cgi?id=793565
Previously, only gnextstepsettingsbackend.c was taken into account. https://bugzilla.gnome.org/show_bug.cgi?id=793565