netsnmp: fix static build

Exclude the ucd-snmp/dlmod MIB module when building statically.

Fixes:
http://autobuild.buildroot.net/results/782/7829e74f83c3c4a1f45e049ec9f9d6b00e002c1c/
http://autobuild.buildroot.net/results/5df/5df28482dfa3d7c54d1e4ae3e1d373bd8e9a2265/
http://autobuild.buildroot.net/results/4b7/4b7f3c15a4f4c4f4457e78767f5526c044d6dc1d/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>