Move vnstat into "backports" directory

Making it clear that it was backported from a more recent release of the
networking layer.

BugzID: 74466

conf/layer.conf

@@ -5,6 +5,8 @@ BBPATH .= ":${LAYERDIR}"
 BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \
             ${LAYERDIR}/recipes-*/*/*.bbappend \
             ${LAYERDIR}/bbappends/*/recipes-*/*/*.bbappend \
+            ${LAYERDIR}/backports/*/recipes-*/*/*.bb \
+            ${LAYERDIR}/backports/*/recipes-*/*/*.bbappend \
         "
 
 BBFILE_COLLECTIONS += "netmodule-distro"