windows: make tests work with only feature backend-mmap

The tests module in mmap/windows.rs assumed that `crate::bitmap` was
always available, but it is not. Hide the imports / usage of that crate
behind appropriate cfgs.

Fixes: 4757a4a ("do not backdoor-enable backend-bitmap in unittests")
Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

src/mmap/windows.rs

@@ -245,6 +245,7 @@ impl<B> Drop for MmapRegion<B> {
 mod tests {
     use std::os::windows::io::FromRawHandle;
 
+    #[cfg(feature = "backend-bitmap")]
     use crate::bitmap::AtomicBitmap;
     use crate::guest_memory::FileOffset;
     use crate::mmap::windows::INVALID_HANDLE_VALUE;
@@ -260,6 +261,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(feature = "backend-bitmap")]
     fn test_dirty_tracking() {
         // Using the `crate` prefix because we aliased `MmapRegion` to `MmapRegion<()>` for
         // the rest of the unit tests above.