tools: fix missing [[fallthrough]] in js2c

The latest clang warns about this.

PR-URL: nodejs#51845
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

tools/js2c.cc

@@ -603,6 +603,7 @@ bool Simplify(const std::vector<char>& code,
           simplified_count++;
           break;
         }
+        [[fallthrough]];
       }
       default: {
         simplified->push_back(code[i]);