res_snmp: Declare RONLY if net-snmp headers do not.

Some net-snmp builds do not provide the RONLY declare, only
NETSNMP_OLDAPI_RONLY.  Map RONLY to NETSNMP_OLDAPI_RONLY to get around
this error.

Change-Id: Ida5c7ad9406515825485c4d3b4a34fd6ad0da577