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/sssd.c

@@ -70,6 +70,6 @@ int sssd_flush_cache (int dbflags)
 	return 0;
 }
 #else				/* USE_SSSD */
-extern int errno;		/* warning: ANSI C forbids an empty source file */
+extern int ISO_C_forbids_an_empty_translation_unit;
 #endif				/* USE_SSSD */