Silence "Variable Unused" warnings when assertions are blocked (Release builds)

Only warning left is due to an inexistent method `setContentScaleFactor:` in [GPUImageView.m#L430](https://github.com/BradLarson/GPUImage/blob/6e7fc67cbd6fdd4a43c564539a2614ad0e58fbf7/framework/Source/iOS/GPUImageView.m#L430).