Fix cannot infer return type for closure

This commit will add the type to the dateFormatter property declaration
in the OnlineSubtitle class.

This corrects a "cannot infer return type for closure with multiple
statements" compilation error under older versions of Xcode. Normally
IINA only supports the latest version of Xcode, however at the moment
we are trying to maintain compatibility with Xcode 14 to support
building for older versions of macOS that are not supported by
Xcode 15.

iina/OpenSubSubtitle.swift

@@ -16,7 +16,7 @@ import PromiseKit
 class OpenSub {
   final class Subtitle: OnlineSubtitle {
 
-    private static let dateFormatter = {
+    private static let dateFormatter: DateFormatter = {
       let dateFormatter = DateFormatter()
       dateFormatter.dateStyle = .medium
       dateFormatter.timeStyle = .medium