versioner: add support for preprocessing headers.

Bug: http://b/30170081
Change-Id: I9b662ca9e0fa3a1f9c0211594e851f5f9bef7266
diff --git a/tools/versioner/src/Utils.h b/tools/versioner/src/Utils.h
index c79c97e..8d8415f 100644
--- a/tools/versioner/src/Utils.h
+++ b/tools/versioner/src/Utils.h
@@ -36,7 +36,7 @@
 static std::string Join(Collection c, const std::string& delimiter = ", ") {
   std::string result;
   for (const auto& item : c) {
-    using std::to_string;
+    using namespace std;
     result.append(to_string(item));
     result.append(delimiter);
   }