x86: Fix warning in pvclock.c

when building 32-bit, I see this ..
arch/x86/kernel/pvclock.c:63:7: warning: "__x86_64__" is not defined

Signed-off-by: Dave Jones <davej@redhat.com>
LKML-Reference: <20090713201437.GA12165@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>