[processing] added missing import (fixes #16181)

(backported from 84bef5f; followup b003cbe)

Author: jef-n

python/plugins/processing/algs/grass/GrassAlgorithm.py

@@ -31,7 +31,7 @@
 import importlib
 import re
 
-from PyQt4.QtCore import QCoreApplication
+from PyQt4.QtCore import QCoreApplication, QUrl
 from PyQt4.QtGui import QIcon
 
 from qgis.core import QgsRasterLayer