Fix compilation when UFFDIO_REGISTER is not set.

Signed-off-by: Pierre Labatut <plabatut@google.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

subprojects/libvhost-user/libvhost-user.c

@@ -631,9 +631,9 @@ static bool
 generate_faults(VuDev *dev) {
     unsigned int i;
     for (i = 0; i < dev->nregions; i++) {
+#ifdef UFFDIO_REGISTER
         VuDevRegion *dev_region = &dev->regions[i];
         int ret;
-#ifdef UFFDIO_REGISTER
         struct uffdio_register reg_struct;
 
         /*