headerbar: fix a rounded corner.
headerbar: fix a rounded corner.

DoctorJellyface/gnome-themes-standard,rgcjonas/adwaita-semidark,rgcjonas/adwaita-semidark,rgcjonas/adwaita-semidark,Distrotech/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,Distrotech/gnome-themes-standard,rgcjonas/adwaita-semidark