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>

libmisc/limits.c

@@ -550,6 +550,6 @@ void setup_limits (const struct passwd *info)
 }
 
 #else				/* !USE_PAM */
-extern int errno;		/* warning: ANSI C forbids an empty source file */
+extern int ISO_C_forbids_an_empty_translation_unit;
 #endif				/* !USE_PAM */