ForbiddenFunctionsSniff will ignore function calls preceded by a T_NS_SEPARATOR

The following code will successfully give an error/warning:

    sizeof('abc');

While this will work exactly the same in PHP, but will not give an error/warning:

    \sizeof('abc');

This pull request would fix that, while still not giving an error/warning on the following code:

    \my\ns\sizeof('abc');

Unit tests and code style sniffs all pass.