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>

BUILD.gn

@@ -71,6 +71,7 @@ config("tint_public_config") {
   include_dirs = [
     "${tint_root_dir}/",
     "${tint_root_dir}/include/",
+    "${tint_spirv_headers_dir}/include",
   ]
 }