tools: fix missing [[fallthrough]] in js2c

The latest clang warns about this.

PR-URL: #51845
Reviewed-By: Chengzhong Wu <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Mohammed Keyvanzadeh <[email protected]>
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

tools/js2c.cc

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