ptp-helper: Allow unused Stderr export

It's not used when building tests and would otherwise cause a warning.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5872>

subprojects/gstreamer/libs/gst/helpers/ptp/io.rs

@@ -1327,7 +1327,10 @@ mod imp {
     }
 }
 
-pub use self::imp::{Poll, Stderr, Stdin, Stdout};
+// `Stderr` is not used when tests are enabled as no logging is performed to stderr.
+#[cfg_attr(test, allow(unused_imports))]
+pub use self::imp::Stderr;
+pub use self::imp::{Poll, Stdin, Stdout};
 
 #[cfg(test)]
 mod test {