Adwaita: Add missing rule for fullscreen headerbar

Adwaita: Add missing rule for fullscreen headerbar

Another selector forces round corners for headerbars in a stack, and it
has higher priority than the selector covering the non-stack case from
commit 712a8adbd9a11fcebce0269562e9a028a17fa862. Totem’s MainToolbar
happens to be in a stack, and we should maintain symmetry here anyway.

So, as window classes .maximized and .tiled are excluded from this other
selector, the newly handled .fullscreen case must be excluded here also.

https://bugzilla.gnome.org/show_bug.cgi?id=770513