buildroot

janus-gateway: make video call plugin optional

Currently janus-gateway is built by default with the video call plugin.
This causes a example config file to be placed on the target filing system
that the user may not need or want, creates a 47K .so
file on the target directory, and janus will also generate a warning when
ran if the plugin does not have a matching configuration file setup by
the user, as it will not use the example config file by default.

This patch makes the video call plugin optional.

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