add availability check to UIImage+SFSymbol

Author: kernjackson

Sources/SFSymbol/UIImage+SFSymbol.swift

@@ -8,6 +8,7 @@
 #if canImport(UIKit)
 import UIKit
 
+@available(iOS 13.0, *)
 public extension UIImage {
     convenience init?(symbol: SFSymbol) {
         self.init(systemName: symbol.rawValue)