buffer: wrap EXTERN in DEPRECATED

The GIT_EXTERN macro needs to provide order-specific attributes; update
users of the GIT_DEPRECATED macro to allow for that.

include/git2/buffer.h

@@ -81,7 +81,7 @@ GIT_EXTERN(void) git_buf_dispose(git_buf *buffer);
  *
  * This function is going to be removed in v0.30.0.
  */
-GIT_EXTERN(void) GIT_DEPRECATED(git_buf_free)(git_buf *buffer);
+GIT_DEPRECATED(GIT_EXTERN(void)) git_buf_free(git_buf *buffer);
 
 /**
  * Resize the buffer allocation to make more space.