Fix warning of function only used in integration tests

Author: Bromeon

gdnative-core/src/core_types/transform2d.rs

@@ -281,6 +281,7 @@ impl std::ops::Mul<Transform2D> for Transform2D {
     }
 }
 
+#[cfg(feature = "gd_test")]
 fn test_transform2d_behavior_impl() {
     let api = crate::private::get_api();