lguest: override sched_clock

Guests currently use the default scheduler clock: this means they
always use jiffies even if TSC is actually available.  It doesn't make
any noticeable difference here, but it's a better thing to do.

Also remove commented-out asm/sched-clock.h from -mm tree.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>