Fix missing std::io::Read import for Windows build in ec_encoder

The #[cfg(not(unix))] fallback path uses f.read() which requires
the Read trait to be in scope.

seaweed-volume/src/storage/erasure_coding/ec_encoder.rs

@@ -6,7 +6,7 @@
 use std::fs::File;
 use std::io;
 #[cfg(not(unix))]
-use std::io::{Seek, SeekFrom};
+use std::io::{Read, Seek, SeekFrom};
 
 use reed_solomon_erasure::galois_8::ReedSolomon;