StyleProviderPrivate: mark internal variable as static
StyleProviderPrivate: mark internal variable as static

Otherwise, it could conflict with other symbols with the same
name exported by a different shared library, with undefined
effects.

https://bugzilla.gnome.org/show_bug.cgi?id=674577

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