Fix report_vuln duplicate VulnRef record creation

Multiple calls to report_vuln for the same vuln results in the creation
of duplicate VulnRef records, except for the CVE refs

lib/msf/core/db_manager/vuln.rb

@@ -46,7 +46,7 @@ module Msf::DBManager::Vuln
 
   def find_vuln_by_refs(refs, host, service=nil)
     ref_ids = refs.find_all { |ref| ref.name.starts_with? 'CVE-'}
-    relation = host.vulns.includes(:refs)
+    relation = host.vulns.joins(:refs)
     if !service.try(:id).nil?
       return relation.where(service_id: service.try(:id), refs: { id: ref_ids}).first
     end