chore(frontends/lean): fix style

src/frontends/lean/pp.h

@@ -5,6 +5,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Author: Leonardo de Moura
 */
 #pragma once
+#include <string>
 #include <utility>
 #include <limits>
 #include "util/pair.h"

src/frontends/lean/scanner.cpp

@@ -160,7 +160,7 @@ auto scanner::read_quoted_symbol() -> token_kind {
         } else if (c != '\"' && c != '\n' && c != '\t') {
             m_buffer += c;
         } else {
-            // TODO: intra-token space
+            // TODO(Leo): intra-token space
             throw_exception("invalid quoted symbol, invalid character");
         }
     }