[tint][build] Guard WGSL writer with tint_build_wgsl_writer

Change-Id: Id188456ada3fae3abd7931c453f1d958e4e9b9a4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/155444
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: James Price <jrprice@google.com>
diff --git a/src/tint/lang/wgsl/writer/BUILD.cfg b/src/tint/lang/wgsl/writer/BUILD.cfg
new file mode 100644
index 0000000..80e6a14
--- /dev/null
+++ b/src/tint/lang/wgsl/writer/BUILD.cfg
@@ -0,0 +1,3 @@
+{
+    "condition": "tint_build_wgsl_writer"
+}