x86/platform: protect XENPF_get_dom0_console if CONFIG_VIDEO not set

A build failure [1] is observed if CONFIG_VGA (and thus CONFIG_VIDEO) is
not set. This is because XENPF_get_dom0_console cmd of platform hypercall
makes a call to fill_console_start_info, which is defined in video/vga.c
and built only if CONFIG_VGA is set.

To fix this issue, protect XENPF_get_dom0_console with CONFIG_VIDEO
ifdefery.

[1]:
ld: prelink.o: in function `do_platform_op':
(.text.do_platform_op+0x1a7): undefined reference to `fill_console_start_info'

Fixes: 4dd160583c ("x86/platform: introduce hypercall to get initial video console settings")
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/platform_hypercall.c

@@ -839,6 +839,7 @@ ret_t do_platform_op(
     }
     break;
 
+#ifdef CONFIG_VIDEO
     case XENPF_get_dom0_console:
         if ( !fill_console_start_info(&op->u.dom0_console) )
         {
@@ -849,6 +850,7 @@ ret_t do_platform_op(
         if ( copy_field_to_guest(u_xenpf_op, op, u.dom0_console) )
             ret = -EFAULT;
         break;
+#endif
 
     default:
         ret = -ENOSYS;