Fix Windows linker warning in components/policy/generated.lib

BUG=643470

Review-Url: https://codereview.chromium.org/2359033002
Cr-Commit-Position: refs/heads/master@{#420343}
diff --git a/components/policy/BUILD.gn b/components/policy/BUILD.gn
index e5243c85f..580ff43 100644
--- a/components/policy/BUILD.gn
+++ b/components/policy/BUILD.gn
@@ -166,6 +166,7 @@
       risk_tag_header_path,
     ]
 
+    defines = [ "POLICY_COMPONENT_IMPLEMENTATION" ]
     public_deps = [
       ":cloud_policy_code_generate",
       ":cloud_policy_proto_generated_compile",