snmp__if_combined: provide CDEFs directives for the combined graph as well.

Borrowed data is raw, so in this case we want to make sure it's expressed in bits rather than octects.

plugins/snmp/snmp__if_combined

@@ -613,8 +613,10 @@ END
       print <<END;
 recv$if.label $alias
 recv$if.graph no
+recv$if.cdef recv$if,8,*
 send$if.label $alias
 send$if.negative recv$if
+send$if.cdef send$if,8,*
 END
     }
 }