useradd_base: Fix sed command line for passwd-expire

useradd_base: Fix sed command line for passwd-expire

A previous commit tried to add the --follow-symlinks option to
the perform_passwd_expire function in useradd_base.bbclass, however it used
a single -.

This is interpreted as --file=ollow-symlinks which results in...

sed: couldn't open file ollow-symlinks: No such file or directory

and...

ERROR: <image name>: passwd --expire operation did not succeed.

Fix by adding the missing -

Signed-off-by:  Adam Johnston <adam.johnston@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>