Define php_ini_scanned_files [closes #3550]

Summary: 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)
```
Closes https://github.com/facebook/hhvm/pull/3872

Reviewed By: @paulbiss

Differential Revision: D1585754