wpa_supplicant: fix static build with EAP enabled

Disable EAP-TNC for static build because its implementation uses dlopen().

 Fixes:
http://autobuild.buildroot.net/results/e21/e21705ae690ab899b6f00e4a15833abef5e54e0e/



[Peter: add comment explaining why]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>