Fix code for x86_64 v0.9.5 update

Author: phil-opp

src/allocator.rs

@@ -17,7 +17,7 @@ static ALLOCATOR: LockedHeap = LockedHeap::empty();
 pub fn init_heap(
     mapper: &mut impl Mapper<Size4KiB>,
     frame_allocator: &mut impl FrameAllocator<Size4KiB>,
-) -> Result<(), MapToError> {
+) -> Result<(), MapToError<Size4KiB>> {
     let page_range = {
         let heap_start = VirtAddr::new(HEAP_START as u64);
         let heap_end = heap_start + HEAP_SIZE - 1u64;