Introduce GIT_CALLBACK macro to enforce cdecl

Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.

The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.

include/git2/revwalk.h

@@ -274,7 +274,7 @@ GIT_EXTERN(git_repository *) git_revwalk_repository(git_revwalk *walk);
  * @param commit_id oid of Commit
  * @param payload User-specified pointer to data to be passed as data payload
  */
-typedef int(*git_revwalk_hide_cb)(
+typedef int GIT_CALLBACK(git_revwalk_hide_cb)(
 	const git_oid *commit_id,
 	void *payload);