Added an update_icon() to lockbox.dm so the sprite properly updates when the lockboxes are openned/closed by an EMP

Before then, it would keep using the "locked" sprite even when the EMP successfully unlocked the lockbox.

I should add that the emp_act function in lockbox.dm was calling for update_icon(). It would properly update the icon of /lockbox/vials which has its own update_icon() in fancy.dm but wouldn't find one for regular lockboxes.