x86: fix sparse warning in xen/time.c

Use xen_khz to denote xen_specific clock speed.  Avoid shadowing
cpu_khz.

arch/x86/xen/time.c:220:6: warning: symbol 'cpu_khz' shadows an earlier one
include/asm/tsc.h:17:21: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>