Add missing override

R=bclayton@google.com, rharrison@chromium.org

Change-Id: I3daffbb7bf9b9e74e4bbadca7a62af5ceb6f2475
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/33121
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/writer/spirv/test_helper.h

@@ -34,7 +34,7 @@ template <typename BASE>
 class TestHelperBase : public ast::BuilderWithContext, public BASE {
  public:
   TestHelperBase() : td(ctx, &mod), b(ctx, &mod) {}
-  ~TestHelperBase() = default;
+  ~TestHelperBase() override = default;
 
   /// The module
   ast::Module mod;