janus-gateway: disable for static build

janus-gateway requires dlopen().

Fixes:
http://autobuild.buildroot.net/results/092/092aac2b8abe0edd868a7e0cc59cdaaf28ca4ad5/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>