Add imports in tag_hash macro

melib/src/backends.rs

@@ -22,6 +22,8 @@
 #[macro_export]
 macro_rules! tag_hash {
     ($tag:ident) => {{
+        use std::collections::hash_map::DefaultHasher;
+        use std::hash::Hasher;
         let mut hasher = DefaultHasher::new();
         hasher.write($tag.as_bytes());
         hasher.finish()