AI: added hidden= as valid non-aspect key

Should have added this in b2df837. Should fix the warning
1d84d83 attempted to fix.

src/ai/configuration.cpp

@@ -256,7 +256,7 @@ bool configuration::parse_side_config(side_number side, const config& original_c
 
 }
 
-static const std::set<std::string> non_aspect_attributes {"turns", "time_of_day", "engine", "ai_algorithm", "id", "description"};
+static const std::set<std::string> non_aspect_attributes {"turns", "time_of_day", "engine", "ai_algorithm", "id", "description", "hidden"};
 static const std::set<std::string> just_copy_tags {"engine", "stage", "aspect", "goal", "modify_ai"};
 static const std::set<std::string> old_goal_tags {"target", "target_location", "protect_unit", "protect_location"};