Heap: Add a new block to the end of the linked list

*facepalm*

This was causing page faults when having two blocks and the first one (oldest one) being freed first.

kernel/src/memory/Heap.cpp

@@ -222,6 +222,7 @@ Result<void*> kmalloc(usize size)
         block->status = BLOCK_START_MEM | BLOCK_END_MEM;
         block->next = nullptr;
         block->last = heap_end;
+        heap_end->next = block;
 
         heap_end = block;
     }