Update Capabilities.swift so Xcode 13 can build the app (huggingface#66)

Update Capabilities.swift

Changed constant deviceSupportsQuantization to include word "return" so the project could be built using Xcode 13

Author: mattusi

Diffusion-macOS/Capabilities.swift

@@ -13,9 +13,9 @@ let deviceHas6GBOrMore = true
 
 let deviceSupportsQuantization = {
     if #available(macOS 14, *) {
-        true
+        return true
     } else {
-        false
+        return false
     }
 }()