quagga: fix build breakage without snmp

Quagga no longer supports explicitly disabling snmp, and what's worse,
it interprets --disable-snmp as --enable-snmp.
Remove the disable configure option.
While at it add a comment about net-snmp requiring AgentX support for
quagga to build properly as suggested by Arnout.
Fixes:
http://autobuild.buildroot.net/results/a9bce6dd8065ddb1850a4e14f0e4eb257ee89286/



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