RCA AI: fix a bug that kept surrounded units from attacking

In the equation below the line changed, support is a measure of how
many AI units can come and aid the attacking unit.  For a surrounded
unit, support=0, and thus the attack rating (value) was previously
always zero, meaning that the unit never attacked.
This fixes bug #21334:

src/ai/default/attack.cpp

@@ -308,7 +308,7 @@ double attack_analysis::rating(double aggression, const readonly_context& ai_obj
                }
         }
 
-	if(!leader_threat && vulnerability*terrain_quality > 0.0) {
+	if(!leader_threat && vulnerability*terrain_quality > 0.0  && !is_surrounded) {
 		value *= support/(vulnerability*terrain_quality);
 	}