spinbutton: fix style context path for internal buttons
spinbutton: fix style context path for internal buttons

We were adding one child too much to the style context path when
generating it for the internal buttons, which in turn caused sibling
selectors from the theme such as :first-child to apply to both buttons
under certain circumstances. Spotted by Lapo Calamandrei.

msteinert/gtk,davidt/gtk,jigpu/gtk,Lyude/gtk-,Lyude/gtk-,Adamovskiy/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,jadahl/gtk,Sidnioulz/SandboxGtk,chergert/gtk,jadahl/gtk,ebassi/gtk,jigpu/gtk,Distrotech/gtk2,Sidnioulz/SandboxGtk,msteinert/gtk,Distrotech/gtk2,jigpu/gtk,davidt/gtk,jessevdk/gtk,ahodesuka/gtk,Lyude/gtk-,Distrotech/gtk2,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,chergert/gtk,jigpu/gtk,ahodesuka/gtk,jessevdk/gtk,alexlarsson/gtk,Lyude/gtk-,msteinert/gtk,jessevdk/gtk,jigpu/gtk,msteinert/gtk,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,ebassi/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,jessevdk/gtk,ahodesuka/gtk,ahodesuka/gtk,Adamovskiy/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jadahl/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,davidt/gtk,davidgumberg/gtk,jigpu/gtk,chergert/gtk,alexlarsson/gtk,ebassi/gtk,jadahl/gtk,ebassi/gtk,alexlarsson/gtk,Lyude/gtk-,ebassi/gtk,davidgumberg/gtk,bratsche/gtk-,davidgumberg/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,bratsche/gtk-,Adamovskiy/gtk,jessevdk/gtk,alexlarsson/gtk,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,Adamovskiy/gtk,grubersjoe/adwaita,Distrotech/gtk2,jessevdk/gtk,davidt/gtk,Sidnioulz/SandboxGtk,chergert/gtk,Lyude/gtk-,jadahl/gtk,msteinert/gtk,davidgumberg/gtk,bratsche/gtk-,Adamovskiy/gtk,Lyude/gtk-,bratsche/gtk-,ebassi/gtk,chergert/gtk,bratsche/gtk-,Distrotech/gtk2,Adamovskiy/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,Lyude/gtk-,chergert/gtk,ahodesuka/gtk,Distrotech/gtk2,ahodesuka/gtk,ahodesuka/gtk,ahodesuka/gtk,grubersjoe/adwaita,davidgumberg/gtk,msteinert/gtk,davidgumberg/gtk,davidt/gtk,davidt/gtk,jessevdk/gtk