put /usr/include on include search path

dtool/src/interrogate/interrogate.cxx

@@ -425,6 +425,8 @@ main(int argc, char *argv[]) {
     exit(1);
   }
 
+  parser._system_include_path.append_directory("/usr/include");
+
   output_code_filename.set_text();
   output_data_filename.set_text();
   output_data_basename = output_data_filename.get_basename();