Fixed mtime reading from OpenStack API

The API seems to return floating point values, which prevents
the hasUpdated() check to work and causes the scanner to rescan
everything every time.

Backport of fa00a18 from master

apps/files_external/lib/swift.php

@@ -245,6 +245,10 @@ class Swift extends \OC\Files\Storage\Common {
 			$mtime = $object->extra_headers['X-Object-Meta-Timestamp'];
 		}
 
+		if (!empty($mtime)) {
+			$mtime = floor($mtime);
+		}
+
 		$stat = array();
 		$stat['size'] = $object->content_length;
 		$stat['mtime'] = $mtime;