m68k: use the MMU version of unistd.h for all m68k platforms

The MMU version of unistd.h can be use on non-MMU platrorms as well.
Without a single file "make headers_install" is broken for m68k
(since each of the sub-varients of unistd.h are not installed).

Signed-off-by: Greg Ungerer <gerg@uclinux.org>