Fixes private func selector error.

1. Launch App
2. Leave app and change dynamic text size
3. Go back to app -> crash.

Based on Apple comments in the dev forums:

>"Private members are not implicitly marked with @objc by the compiler; you must do so yourself to expose them to the Objective-C runtime"
 
[Link](https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithObjective-CAPIs.html#//apple_ref/doc/uid/TP40014216-CH4-XID_26)

Changing `private func onContentSizeChange(notification: NSNotification)` to   
`@objc private func onContentSizeChange(notification: NSNotification)`

resolves the crash.