Fix libgit2 build for iOS

- fixes libgit2 build
- objective-git build still fails (fixed now, see comments below)

    Classes/GTDiff.m:61:2: No visible @interface for 'GTRepository' declares the selector 'isEqualTo:'

commenting out the NSParameterAssert "fixes" the whole build.
Seems we need to implement an 'isEqualTo:' method here. Just curious why this is not an issue on osx...

Hope that helps