mkosi: set CONFIG_AUTOFS_FS rather than CONFIG_AUTOFS4_FS

mkosi: set CONFIG_AUTOFS_FS rather than CONFIG_AUTOFS4_FS

Since Linux 4.18 CONFIG_AUTOFS4_FS just enables CONFIG_AUTOFS_FS, its
description in fs/autofs/Kconfig reads:

	   This name exists for people to just automatically pick up the
	   new name of the autofs Kconfig option. All it does is select
	   the new option name.

	   It will go away in a release or two as people have
	   transitioned to just plain AUTOFS_FS.