Fixed Windows build of www_form_url_encoded.

PiperOrigin-RevId: 248638848

Author: smiles

app/rest/www_form_url_encoded.h

@@ -31,7 +31,8 @@ class WwwFormUrlEncoded {
   // Form item.
   struct Item {
     Item() {}
-    Item(const string& key_, const string& value_) : key(key_), value(value_) {}
+    Item(const std::string& key_, const std::string& value_)
+        : key(key_), value(value_) {}
 
     std::string key;
     std::string value;