title: Cpplint asks to add header file
body: <p>I have some code that uses an std::string in a file named executor.cpp. In the corresponding header file, executor.h I include <code>#include &lt;string&gt;</code>. However, running cpplint on executor.cpp gives me the message that says :-</p>
  
  <p><code>dec-session/executor.cpp:15:  Add #include &lt;string&gt; for string  [build/include_what_you_use] [4]</code></p>
  
  <p>What am I doing wrong? Should I be including it in the executor.h?</p>