[niepce] rust: Fix a warning

[Hubert Figuière <hub src gnome org>]

commit 1b664693da901f2d1979b2fc29908f50ef96d05e
Author: Hubert Figuière <hub figuiere net>
Date:   Wed Oct 4 23:47:25 2017 -0400

    rust: Fix a warning

 src/engine/library/notification.rs |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/engine/library/notification.rs b/src/engine/library/notification.rs
index cebe263..0487274 100644
--- a/src/engine/library/notification.rs
+++ b/src/engine/library/notification.rs
@@ -103,6 +103,7 @@ pub enum Notification {
 }
 
 #[cfg(not(test))]
+#[allow(improper_ctypes)]
 extern "C" {
     // actually a *mut Notification
     pub fn engine_library_notify(notif_id: u64, n: *mut Notification);