Move `@objc` mark to function

This resolves the library revolution error when building with Swift 5.1.3

Sources/Extensions/ImageView+Kingfisher.swift

@@ -403,8 +403,8 @@ extension KingfisherWrapper where Base: KFCrossPlatformImageView {
 }
 
 
-@objc extension KFCrossPlatformImageView {
-    func shouldPreloadAllAnimation() -> Bool { return true }
+extension KFCrossPlatformImageView {
+    @objc func shouldPreloadAllAnimation() -> Bool { return true }
 }
 
 extension KingfisherWrapper where Base: KFCrossPlatformImageView {