lguest should depend on CONFIG_FUTEX

It uses get_futex_key().
Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/lguest/Kconfig

 config LGUEST
 	tristate "Linux hypervisor example code"
-	depends on X86 && PARAVIRT && EXPERIMENTAL && !X86_PAE
+	depends on X86 && PARAVIRT && EXPERIMENTAL && !X86_PAE && FUTEX
 	select LGUEST_GUEST
 	select HVC_DRIVER
 	---help---