cp: make cp -a not fail on Windows

Before this commit, `cp -a` would terminate with a non-zero status
code on Windows because there are no extended attributes (xattr) to
copy. However, the GNU documentation for cp states

> Try to preserve SELinux security context and extended attributes
> (xattr), but ignore any failure to do that and print no
> corresponding diagnostic.

so it seems reasonable to do nothing instead of exiting with an error
in this case.

src/uu/cp/src/cp.rs

@@ -1146,7 +1146,16 @@ fn copy_attribute(source: &Path, dest: &Path, attribute: &Attribute) -> CopyResu
             }
             #[cfg(not(unix))]
             {
-                return Err("XAttrs are only supported on unix.".to_string().into());
+                // The documentation for GNU cp states:
+                //
+                // > Try to preserve SELinux security context and
+                // > extended attributes (xattr), but ignore any failure
+                // > to do that and print no corresponding diagnostic.
+                //
+                // so we simply do nothing here.
+                //
+                // TODO Silently ignore failures in the `#[cfg(unix)]`
+                // block instead of terminating immediately on errors.
             }
         }
     };