Fix build on GCC: Add missing include

Change-Id: Id5c8701a673c8fececb83e94dab909272d2f6c00
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/65341
Commit-Queue: Ben Clayton <bclayton@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>

generator/templates/dawn_native/ObjectType.h

@@ -17,6 +17,8 @@
 
 #include "common/ityp_array.h"
 
+#include <cstdint>
+
 namespace dawn_native {
 
     enum class ObjectType : uint32_t {