Change platform in xwindows_runlevel_target

The rule xwindows_runlevel_target should be applicable during
bootable container image build. The built image should contain
a correct systemd default target because once the image is booted
this target will be applied. Therefore we are extending the
rule applicability also to bootable containers.

linux_os/guide/services/xwindows/disabling_xwindows/xwindows_runlevel_target/rule.yml

@@ -53,4 +53,4 @@ fixtext: |-
 
 srg_requirement: 'The graphical display manager must not be the default target on {{{ full_name }}} unless approved.'
 
-platform: machine
+platform: system_with_kernel