conntrack-tools: uses dlopen(), not available on static builds

Fixes:
http://autobuild.buildroot.net/results/fff/fff4865c9bb8ca42d3cece903d0fc3daea504f67/
http://autobuild.buildroot.net/results/742/74222a44ad168148c18af479878d8f848d3d6a73/
http://autobuild.buildroot.net/results/470/470197fad2402be9222b8abf96dd45c341a850e9/
http://autobuild.buildroot.net/results/ab1/ab1ae40f1862300cd7667b7bd03cc59d7455a769/



And many more.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>