Fix possible invalid redeclaration error

[XmasRights]

• Array extension in AnimatedImageView.swift uses default access level for subscript(safe:) method
• This is a common Array extension method to make, and using this current access control level can lead to Invalid Redeclaration of function errors in Xcode

Steps to reproduce:

1. Create a new, empty Swift project, and import the Kingfisher library
2. Add the same Array extension in AppDelegate.swift

• Note: this error will occur regardless of the access control level of any duplicate extension (so even a fileprivate subscript(safe:) elsewhere will trigger the error)

[onevcat]

Great catch!