The PCMachineState type is only used under hw/i386/.
We don't need to forward-declare it for all architectures,
restrict it to the X86 one.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200908155530.249806-7-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>