Fixing $forbiddenFunctions visibility

Changing visibility of $forbiddenFunctions to correct the error below:

PHP Fatal error:  Access level to PHPCompatibility_Sniffs_PHP_DeprecatedFunctionsSniff::$forbiddenFunctions must be public (as in class Generic_Sniffs_PHP_ForbiddenFunctionsSniff) in /usr/share/pear/PHP/CodeSniffer/Standards/PHPCompatibility-master/Sniffs/PHP/DeprecatedFunctionsSniff.php on line 358