Fix -Wmissing-field-initializers in playerdat.cpp

```
playerdat.cpp:262:1: warning: missing initializer for member ‘devilution::PlayerData::skill’ [-Wmissing-field-initializers]
  262 | };
```

Source/playerdat.hpp

@@ -28,7 +28,7 @@ struct PlayerData {
 	/* Class Name */
 	const char *className;
 	/* Class Skill */
-	SpellID skill;
+	SpellID skill = SpellID::Null;
 };
 
 struct ClassAttributes {