Added initialization check for iOS Guide

I added an initialization check in iOS's `Guide.Initialise` to make sure that calling Guide.Initialise a second time is safe. As [discussed here](https://github.com/mono/MonoGame/issues/2512), iOS is handling how/when the Guide gets initialized a bit differently than the other platforms, and although it only seems to get called for once right now, it seems prudent to make sure that the call to Initialise is a bit safer in case that ever changes.

Note: GitHub's diff on this file seems to be freaking out a bit (20 additions!), but really all that's changed is there's a `if (!_isInitialised)` check around the initialization code block.