Wikidata engine: minor change of the SPARQL request

The engine can be slow especially when the query won't return any answer.
See https://www.mediawiki.org/wiki/Wikidata_Query_Service/User_Manual/MWAPI#Find_articles_in_Wikipedia_speaking_about_cheese_and_see_which_Wikibase_items_they_correspond_to

Related to #1290

searx/engines/wikidata.py

@@ -65,6 +65,7 @@ WHERE
         mwapi:language "%LANGUAGE%".
         ?item wikibase:apiOutputItem mwapi:item.
   }
+  hint:Prior hint:runFirst "true".
 
   %WHERE%