AndrewReed answered this question on Xamarin forum:
As with quite a few sdk's, sometimes they bundle a folder instead of the static lib,which essentially is the static library. There will be a sizeable file that maybe doesnt have an extension(this is your .a file) so you can rename that to be .a and then include that in a xamarin bindings project. Run the sharpie marker on the header file in the folder which will generate your necessary files. Watch out for one gotcha, delegate is a reserved word in c#, so you may need to go around and change this by adding an @ symbol(to the name) to allow you to use the reserved word. So what I actually did was, created a project in xcode and ran the crashlytics programme, this then added the crashlytics framework to the project. In finder locate that and you will have a structure like so: - run - /Versions --/A ---Crashlytics ---/Headers ----Crashlytics.h
So the /A folder is essentially your static lib, use this and create your binding from that, or in this particular case, just download it from the mono bindings || my repo.