Add missing include patch

This missing in Dawn was being shadowed by spvc also including it.

BUG=dawn:521

Change-Id: I609cfcca74821ad07c0fd9e6fadf2c22fba40df0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/27901
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index f63a82e..3bcfdef 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -71,6 +71,7 @@
   include_dirs = [
     "${tint_root_dir}/",
     "${tint_root_dir}/include/",
+    "${tint_spirv_headers_dir}/include",
   ]
 }