unix/moduos: Include errno.h.

The file `ports/unix/moduos.c` uses `errno` so it needs to include
`errno.h`, otherwise a compiler error can occur.

Signed-off-by: David Lechner <david@pybricks.com>

ports/unix/moduos.c

@@ -25,6 +25,7 @@
  * THE SOFTWARE.
  */
 
+#include <errno.h>
 #include <stdlib.h>
 #include <string.h>