conf/bblayers: add meta-multimedia

The oe/meta-multimedia layer includes support for Gstreamer
RTSP server, which is required in order to build an RTSP servers
using Gstreamer, including the glive application.

Signed-off-by: John Weber <rjohnweber@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>