Define php_ini_scanned_files [closes #3550]

Fixes #3550 - undefined `php_ini_scanned_files()` function.

```php
var_dump(php_ini_scanned_files());
```

Before:
```
$ hhvm test.php
Fatal error: Call to undefined function php_ini_scanned_files() in test.php on line 3
```

After:
```
$ hhvm test.php 
bool(false)
```