[gn] Define tint_has_protobuf to false

This is only needed for clusterfuzz, which will be built from the dawn repo.

Change-Id: I1a2b2d4385901e209a5854bd9f9ffdb084920ac1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/163201
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/build_overrides/tint.gni b/build_overrides/tint.gni
index a4594f2..efb378b 100644
--- a/build_overrides/tint.gni
+++ b/build_overrides/tint.gni
@@ -14,3 +14,4 @@
 
 # This file contains Tint-related overrides.
 tint_build_unittests = true
+tint_has_protobuf = false