sulogin: remove __nonnull__ function attribute

sulogin: remove __nonnull__ function attribute

This change fixes compiler option -Werror=nonnull option warning:

login-utils/sulogin-consoles.c: In function 'append_console':
login-utils/sulogin-consoles.c:324:14: warning: nonnull argument 'consoles'
compared to NULL [-Wnonnull-compare]

The NULL check done with list_empty() looks valid, so  it is best to remove
the function attribute that allows compiler to optimize the check away.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>