Fix log message always saying 0 items were cleared

src/main/java/org/embeddedt/modernfix/mixin/perf/patchouli_deduplicate_books/ClientBookRegistryMixin.java

@@ -53,6 +53,7 @@ public class ClientBookRegistryMixin {
                                         continue;
                                     for (int i = 0; i < items.length; i++) {
                                         if (items[i] != null && items[i].getItem() == Items.AIR) {
+                                            numItemsCleared++;
                                             items[i] = ItemStack.EMPTY;
                                         }
                                     }