Ensure libgit2 calling convention

Ensure that libgit2 is always built with the `__stdcall` convention. As the great and wise @xpaulbettsx [mentioned](https://github.com/libgit2/libgit2/pull/356):

> [U]sing cdecl means that it is impossible to use functions that use callbacks from .NET code, since there is no way to marshal a delegate => function pointer with the cdecl calling convention.

So we definitely want stdcall.