ntp: properly select/filter netsnmp for ntpsnmpd

BR2_PACKAGE_NTP_SNMPD was pushing netsnmp into dependencies but was
never selected, and since netsnmp requires fork it wasn't filtered out
for nommu. Fixes:

http://autobuild.buildroot.net/results/776/7769afe0da09e3f4f96d9a0f4c0febb0c72cc34f/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>