ortp: do not use libsrtp

ortp is not happy with the newly packaged libsrtp implementation, so
let's not use it.

Fixes:

  http://autobuild.buildroot.org/results/ba1/ba1e29c6b5cc7ff7ff4582ec2319598ff9b28bae/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>