PHP 7.1 tempnam warning fix (#2134)

And suggested dir was wrong.

https://bugs.php.net/bug.php?id=69489
```
<b>Notice</b>:  tempnam(): file created in the system's temporary
directory in
<b>/var/www/html/FreshRSS/app/Controllers/importExportController.php</b>
on line <b>800</b><br />
```

app/Controllers/importExportController.php

@@ -797,7 +797,7 @@ class FreshRSS_importExport_Controller extends Minz_ActionController {
 		}
 
 		// From https://stackoverflow.com/questions/1061710/php-zip-files-on-the-fly
-		$zip_file = tempnam('tmp', 'zip');
+		$zip_file = @tempnam('/tmp', 'zip');
 		$zip = new ZipArchive();
 		$zip->open($zip_file, ZipArchive::OVERWRITE);