Add missing import

blog/content/second-edition/posts/10-heap-allocation/index.md

@@ -398,6 +398,13 @@ If we tried to use this heap region now, a page fault would occur since the virt
 ```rust
 // in src/allocator.rs
 
+use x86_64::{
+    structures::paging::{
+        mapper::MapToError, FrameAllocator, Mapper, Page, PageTableFlags, Size4KiB,
+    },
+    VirtAddr,
+};
+
 pub fn init_heap(
     mapper: &mut impl Mapper<Size4KiB>,
     frame_allocator: &mut impl FrameAllocator<Size4KiB>,