w: indicate unknown host when -f is used

The option -f used too print `-' to FROM column when host was not
known. This commit changes behaviour back to what it was earlier.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

w.c

@@ -78,6 +78,8 @@ static void print_host(const char *restrict host, int len, const int fromlen)
 			fputc(*host, stdout);
 			++width;
 		} else {
+			fputc('-', stdout);
+			++width;
 			break;
 		}
 	}