net-snmp: fixup paths in net-snmp-config

Exposed by commit 7e3e8ec0
net-snmp-config goes for -L/usr/lib thus things break since this wins
over the sysroot if we have a host net-snmp library around when we build
packages that link against net-snmp.

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