Fix compile issue with chunk generation.

By: Travis Watkins <amaranth@ubuntu.com>

paper-server/src/main/java/org/bukkit/craftbukkit/generator/CustomChunkGenerator.java

@@ -216,6 +216,8 @@ public class CustomChunkGenerator extends InternalChunkGenerator {
         return "Stronghold".equals(type) && this.strongholdGen != null ? this.strongholdGen.getNearestGeneratedFeature(world, x, y, z) : null;
     }
 
+    public void recreateStructures(int i, int j) {}
+
     public int getLoadedChunks() {
         return 0;
     }