added missing include

Author: jermp

include/parser.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <iostream>
 #include <vector>
 #include <unordered_map>
 #include <algorithm>
@@ -157,4 +158,4 @@ struct parser {
     }
 };
 
-}  // namespace cmd_line_parser
+}  // namespace cmd_line_parser