Don't read from volume when reloading Wii banners

Should make loading cached Wii games that lack banners slightly faster.

Source/Core/DiscIO/VolumeDirectory.h

@@ -43,6 +43,7 @@ public:
 	u16 GetRevision() const override { return 0; }
 	std::string GetInternalName() const override;
 	std::map<IVolume::ELanguage, std::string> GetNames(bool prefer_long) const override;
+	std::vector<u32> GetBanner(int* width, int* height) const override;
 	void SetName(const std::string&);
 
 	u64 GetFSTSize() const override;