kvmclock_create() is only implemented in hw/i386/kvm/clock.h.
Restrict the "hw/kvm/clock.h" header to i386 by moving it to
hw/i386/.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230620083228.88796-3-philmd@linaro.org>