[build] Add missing testonly marker

Bug: BLD-493 #comment Fix topaz case where test_spec() deps on non-testonly
Change-Id: Iad2368014cdfe1820ba90760d943bb116932b9e8
diff --git a/public/dart/zircon/BUILD.gn b/public/dart/zircon/BUILD.gn
index e0da770..ba1607d 100644
--- a/public/dart/zircon/BUILD.gn
+++ b/public/dart/zircon/BUILD.gn
@@ -111,6 +111,7 @@
 
   # This is effectively a host test target.
   copy("test_extract_zircon_constants") {
+    testonly = true
     sources = [
       "test-extract-zircon-constants.sh",
     ]