syscons: scrnmaps: appease -Wmissing-variable-declarations

scrmap is only used in the one compilation unit in all cases, make it static
rather than extern'ing it. There's little benefit, but it's easy to do.

It's unclear how this hasn't failed many builds before now, since it should
have cropped up sometime around deeper hierarchies getting a default WARNS.

MFC after: 3 days