Fix issue with indirect return for C++ member function w/no args

diff --git a/tests/ffi/lib.rs b/tests/ffi/lib.rs
index d4e96fc..51716e0 100644
--- a/tests/ffi/lib.rs
+++ b/tests/ffi/lib.rs
@@ -83,6 +83,8 @@
         fn set(self: &mut C, n: usize) -> usize;
         fn get2(&self) -> usize;
         fn set2(&mut self, n: usize) -> usize;
+        fn set_succeed(&mut self, n: usize) -> Result<usize>;
+        fn get_fail(&mut self) -> Result<usize>;
     }
 
     extern "C" {