package/netsnmp: fix static build with openssl

Use pkg-config to find openssl dependencies such as lz or latomic

Fixes:
 - http://autobuild.buildroot.org/results/8f6fdbf8a21967363b737bc771252bcded4278a9



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>