login: retire use of __FUNCTION__ macro

login: retire use of __FUNCTION__ macro

This fixes following warning about deprecatd __FUNCTION__ that is another
name for __func__ since GCC (pre 2.95, released July 1999).

    login-utils/login.c:860:10: warning: ISO C does not support
    ‘__FUNCTION__’ predefined identifier [-Wpedantic]

Co-Author: Sami Kerola <kerolasa@iki.fi>
Reference: https://c0x.coding-guidelines.com/6.4.2.2.html
Reference: https://github.com/karelzak/util-linux/pull/802
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>