wc: emit '-' in ouput when set on command-line

When stdin is explicitly specified on the command-line with '-', emit it
in the output stats to match GNU wc output.

Fixes #2188.

tests/by-util/test_wc.rs

@@ -36,6 +36,15 @@ fn test_stdin_default() {
         .stdout_is(" 13 109 772\n");
 }
 
+#[test]
+fn test_stdin_explicit() {
+    new_ucmd!()
+        .pipe_in_fixture("lorem_ipsum.txt")
+        .arg("-")
+        .run()
+        .stdout_is(" 13 109 772 -\n");
+}
+
 #[test]
 fn test_utf8() {
     new_ucmd!()