Fix undeclared symbols in maputil.c

Getting the following build error, I think the fix is to use MS_TRUE and MS_FALSE.

    /tmp/mapserver/maputil.c: In function ‘msEvalTextExpressionJSonEscape’:
    /tmp/mapserver/maputil.c:685:54: error: ‘TRUE’ undeclared (first use in this function)
    ...
    /tmp/mapserver/maputil.c: In function ‘msEvalTextExpression’:
    /tmp/mapserver/maputil.c:690:54: error: ‘FALSE’ undeclared (first use in this function)
    ...