um: mark CONFIG_HIGHMEM as broken

Currently CONFIG_HIGHMEM is broken on User Mode Linux.  I'm not sure if it
worked ever.

It doesn't compile and this breaks randomconfig testing.

Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>