NPC: Fix type casting error for gainMeso() method

Number type values might be passed into the `gainMeso()` method in js
scripts, and thus it expects a `gainMeso(Double gain)` method in the
`NPCConversationManager` class.

src/main/java/scripting/npc/NPCConversationManager.java

@@ -278,6 +278,10 @@ public class NPCConversationManager extends AbstractPlayerInteraction {
         getPlayer().gainMeso(gain);
     }
 
+    public void gainMeso(Double gain) {
+        getPlayer().gainMeso(gain.intValue());
+    }
+
     public void gainExp(int gain) {
         getPlayer().gainExp(gain, true, true);
     }