title: When add include netfilter.h i got error that fields 'in','in6' has incomplete type
body: <p>Have simple , empty project without any code. just includes.
  The include linux/netfilter.h cause to compilation error:</p>
  
  <pre><code>In file included from ../src/main.cpp:2:0:
  /usr/include/linux/netfilter.h:65:17: error: field ‘in’ has incomplete type
  /usr/include/linux/netfilter.h:66:18: error: field ‘in6’ has incomplete type
  </code></pre>
  
  <p>As you can see in the simple project i have 2 includes if i remove the iostream include i will get additional error - '/usr/include/linux/sysctl.h:40:2: error: ‘size_t’ does not name a type'</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;linux/netfilter.h&gt;
  
  int main() {
  }
  </code></pre>
  
  <p>if i add include of netinet/in.h before netfilter.h than no errors.</p>
  
  <p>why my simple main program cannot has these compilation errors?</p>
  
  <p>Thank you</p>