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>

hw/i386/xen/meson.build

diff --git a/hw/i386/xen/meson.build b/hw/i386/xen/meson.build
index 3dc4c4f..3f0df8b 100644
--- a/hw/i386/xen/meson.build
+++ b/hw/i386/xen/meson.build
@@ -1,8 +1,10 @@
 i386_ss.add(when: 'CONFIG_XEN', if_true: files(
-  'xen-hvm.c',
   'xen_apic.c',
   'xen_pvdevice.c',
 ))
+i386_ss.add(when: ['CONFIG_XEN', xen], if_true: files(
+  'xen-hvm.c',
+))
 
 i386_ss.add(when: 'CONFIG_XEN_BUS', if_true: files(
   'xen_platform.c',