[gae] Fix go111 build.

Derp in https://chromium-review.googlesource.com/c/3651087. Presubmit
tester do not build go111 and didn't catch this.

R=iannucci@chromium.org

Change-Id: I41de8ccf57bf57b603c01654cf285368e89f0463
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3651089
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
diff --git a/gae/impl/prod/logger_old.go b/gae/impl/prod/logger_old.go
index a70680e..68b6940 100644
--- a/gae/impl/prod/logger_old.go
+++ b/gae/impl/prod/logger_old.go
@@ -20,6 +20,7 @@
 import (
 	"context"
 	"fmt"
+	"net/http"
 
 	"go.chromium.org/luci/common/logging"