videocompare: Fix warning

Fixed following warning:

warning: unreachable pattern
  --> video/videofx/src/videocompare/hashed_image.rs:76:13
   |
76 |             _ => unreachable!(),
   |             ^
   |
   = note: `#[warn(unreachable_patterns)]` on by default

video/videofx/src/videocompare/hashed_image.rs

@@ -73,6 +73,7 @@ impl HasherEngine {
                 let (val, _) = algo.compare(left, right);
                 val.into()
             }
+            #[cfg(feature = "dssim")]
             _ => unreachable!(),
         }
     }