Don't redefine errno(3)

It is Undefined Behavior to declare errno (see NOTES in its manual page).
Instead of using the errno dummy declaration, use one that doesn't need
a comment.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/sgroupio.c

@@ -302,5 +302,5 @@ int sgr_sort ()
 	return commonio_sort_wrt (&gshadow_db, __gr_get_db ());
 }
 #else
-extern int errno;		/* warning: ANSI C forbids an empty source file */
+extern int ISO_C_forbids_an_empty_translation_unit;
 #endif