Bug 1222172 - fix friend declaration in SVGSwitchElement; r=dholbert

clang-cl warns about this one, saying:

  "unqualified friend declaration referring to type outside of the nearest
  enclosing namespace is a Microsoft extension; add a nested name specifier"

It's not completely obvious to me how this is a Microsoft extension if
GCC and clang both manage to compile it, but explicit is better than
implicit, I suppose, so let's add the :: qualifier.

dom/svg/SVGSwitchElement.h

@@ -21,7 +21,7 @@ typedef SVGGraphicsElement SVGSwitchElementBase;
 
 class SVGSwitchElement final : public SVGSwitchElementBase
 {
-  friend class nsSVGSwitchFrame;
+  friend class ::nsSVGSwitchFrame;
 protected:
   friend nsresult (::NS_NewSVGSwitchElement(nsIContent **aResult,
                                             already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo));