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/pam_pass.c

@@ -55,5 +55,5 @@ void do_pam_passwd (const char *user, bool silent, bool change_expired)
 	(void) pam_end (pamh, PAM_SUCCESS);
 }
 #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 */