package/xvisor: use BR2_PYTHON3_HOST_DEPENDENCY to ensure python3 is available

Commit 24e996d14d28d (package/xvisor: fix build without python interpreter)
added a dependency on host-python3 for the d2c.py script, but this script
does not use any non-standard python modules so we can instead use
BR2_PYTHON3_HOST_DEPENDENCY to only build host-python3 if the build host
does not have python3.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 7f08dc612c2a52a297c50bc047d5fabd7ef7317c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/xvisor/xvisor.mk

@@ -11,7 +11,7 @@ XVISOR_LICENSE = GPL-2.0+
 XVISOR_LICENSE_FILES = COPYING
 XVISOR_INSTALL_IMAGES = YES
 XVISOR_INSTALL_TARGET = NO
-XVISOR_DEPENDENCIES = host-bison host-dtc host-flex host-python3
+XVISOR_DEPENDENCIES = host-bison host-dtc host-flex $(BR2_PYTHON3_HOST_DEPENDENCY)
 
 XVISOR_MAKE_TARGETS = all