librsvg-features.h.in: Fix Public Variable Declaration
librsvg-features.h.in: Fix Public Variable Declaration

... on Windows, as public variables need to be decorated for export and
import so that they can be linked to and referred to properly, when
building librsvg as a DLL.  This is the same thing that is done in GLib.

https://bugzilla.gnome.org/show_bug.cgi?id=753555

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg