Fix BUILD.gn builds

Change-Id: I4e54b30497e064061baff2dcc205b68db87bee2c
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25540
Reviewed-by: dan sinclair <dsinclair@chromium.org>
diff --git a/tint_overrides_with_defaults.gni b/tint_overrides_with_defaults.gni
index eaad186..22e698b 100644
--- a/tint_overrides_with_defaults.gni
+++ b/tint_overrides_with_defaults.gni
@@ -61,4 +61,9 @@
   if (!defined(tint_build_msl_writer)) {
     tint_build_msl_writer = false
   }
+
+  # Build the HLSL output writer
+  if (!defined(tint_build_hlsl_writer)) {
+    tint_build_hlsl_writer = false
+  }
 }