Add "import Foundation" to any class that is using @objc

Summary:
This is to prevent the following error that was seen in D33750740 (8044535803)
```
objc attribute used without importing module 'Foundation'
```

Found the files with:
```
git ls-files '*.swift' | xargs ack -l 'objc' | xargs ack -L 'import (Foundation|XCTest)'
```

Note: In D33783526 (4610b07bda) a custom SwiftLint rule was added to detect this but it only works if there aren't any other imports.

Reviewed By: samodom

Differential Revision: D33803396

fbshipit-source-id: 575d7798a77aac30384f8f44c1d45f97d60fe8f7

Sources/FBSDKGamingServicesKit/FBSDKContextDialogPresenter.swift

@@ -8,6 +8,7 @@
 
 import FacebookGamingServices
 import FBSDKCoreKit
+import Foundation
 
 @objcMembers
 open class FBSDKContextDialogPresenter: NSObject {

Sources/FBSDKGamingServicesKit/FBSDKSwitchContextDialog.swift

@@ -8,6 +8,7 @@
 
 import FacebookGamingServices
 import FBSDKCoreKit
+import Foundation
 
 /**
  A dialog to switch the current context through a web view