libcec: not available on pure static builds

libcec needs libdl, so it cannot work on pure static builds.

Fixes:

  http://autobuild.buildroot.org/results/670/670ce46627daaa4ae9ecdb8ffb6f97d0574354d4/



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