hw/i386/xen: Compile 'xen-hvm.c' with Xen CPPFLAGS xen-hvm.c calls xc_set_hvm_param() from <xenctrl.h>, so better compile it with Xen CPPFLAGS. Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk> Message-Id: <20231114143816.71079-19-philmd@linaro.org>