Update NewFunctionsSniff.php

Fixing the visibility of $forbiddenFunctions to correct the error shown below:

PHP Fatal error:  Access level to PHPCompatibility_Sniffs_PHP_NewFunctionsSniff::$forbiddenFunctions must be public (as in class Generic_Sniffs_PHP_ForbiddenFunctionsSniff) in /usr/share/pear/PHP/CodeSniffer/Standards/PHPCompatibility-master/Sniffs/PHP/NewFunctionsSniff.php on line 23