Fix wrong type_spec in getPixelRegionIterator

Fix following bug:
```php
$magick = new Imagick('magick:rose');
$iterator = ImagickPixelIterator::getPixelRegionIterator($magick, 1, 1, 1, 1);
$iterator = ImagickPixelIterator::getPixelRegionIterator($magick);
```

output
```
PHP Warning:  ImagickPixelIterator::getpixelregioniterator() expects exactly 1 parameter, 5 given in /tmp/bug.php on line 3
php: zero region size `Q16' @ error/pixel-iterator.c/NewPixelRegionIterator/430.
```