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>

arch/um/Kconfig.um

@@ -131,7 +131,7 @@ config NR_CPUS
 
 config HIGHMEM
 	bool "Highmem support (EXPERIMENTAL)"
-	depends on !64BIT && EXPERIMENTAL
+	depends on !64BIT && BROKEN
 	default n
 	help
 	  This was used to allow UML to run with big amounts of memory.