Fix missing import & broken build.

That's what I get for editing through Github's web UI...

src/main/java/com/gmail/nossr50/skills/woodcutting/Woodcutting.java

@@ -5,6 +5,7 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
 
+import org.bukkit.Material;
 import org.bukkit.block.BlockFace;
 import org.bukkit.block.BlockState;
 import org.bukkit.inventory.ItemStack;