dawn: Fix clang warning treated as error

Fixes Kokoro builds

Change-Id: I9261da8988091ddc0ae9ab0edd1ccbae64f39417
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/89030
Commit-Queue: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/dawn/native/vulkan/DeviceVk.cpp

@@ -896,7 +896,7 @@ MaybeError Device::CheckDebugLayerAndGenerateErrors() {
 
     AppendDebugLayerMessages(error.get());
 
-    return error;
+    return std::move(error);
 }
 
 void Device::AppendDebugLayerMessages(ErrorData* error) {