quagga: fix build with snmp support

Closes #3277

Quagga's snmp support tries to link with libcrypto from openssl by
default.
That's a bad no no since snmp might have ssl support disabled
and openssl won't be around.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>