gstreamer/message: 2d attempt at printing src for Debug impl

1st attempt in c987bb0c was reverted by 89c4f68f due to
clippy "redundant clone" warning.

gstreamer/src/message.rs

@@ -155,7 +155,14 @@ impl fmt::Debug for MessageRef {
                 CStr::from_ptr(type_).to_str().unwrap()
             })
             .field("seqnum", seqnum)
-            .field("src", &self.get_src().map(|s| s.get_name()))
+            .field(
+                "src",
+                &self
+                    .get_src()
+                    .map(|s| s.get_name())
+                    .as_ref()
+                    .map(glib::GString::as_str),
+            )
             .field("structure", &self.get_structure())
             .finish()
     }