fcl-json: fixed compile pas2js

packages/fcl-json/src/fpjson.pp

@@ -803,8 +803,10 @@ implementation
 
 Uses typinfo;
 
+{$IFNDEF Pas2js}
 const
   HexDigits: array[0..15] of char = '0123456789ABCDEF';
+{$ENDIF}
 
 Resourcestring
   SErrCannotConvertFromNull = 'Cannot convert data from Null value';