x86: pvclock: fix shadowed variable warning

arch/x86/kernel/pvclock.c:102:6: warning: symbol 'tsc_khz' shadows an earlier one
include/asm/tsc.h:18:21: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Avi Kivity <avi@redhat.com>