package/features: add gluon-ebtables-limit-arp to batman feature (#1516)

this activates the package by default when using the batman feature
while still allowing to explicitly remove it like this:

GLUON_SITE_PACKAGES := \
  -gluon-ebtables-limit-arp

package/features

@@ -24,3 +24,6 @@ packages 'web-advanced & autoupdater' \
 
 packages 'status-page & (mesh-batman-adv-14 | mesh-batman-adv-15)' \
         'gluon-status-page-mesh-batman-adv'
+
+packages 'mesh-batman-adv-14 | mesh-batman-adv-15' \
+        'gluon-ebtables-limit-arp'