Fix missing include for std::unique_ptr in NullBinding.cpp

Bug: dawn:92
Change-Id: I9e179bfc7afb92c42d4111676a1c852d6eeae5cb
Reviewed-on: https://dawn-review.googlesource.com/c/3920
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/utils/NullBinding.cpp

@@ -17,6 +17,8 @@
 #include "common/Assert.h"
 #include "dawn_native/NullBackend.h"
 
+#include <memory>
+
 namespace utils {
 
     class NullBinding : public BackendBinding {