bootstrap: add mising libx11-xcb-dev
bootstrap: add mising libx11-xcb-dev

To make gst-plugins-base build happen as GL is
forced to build in the recipe, need to install
this on ubuntu 18.04 to make it work.

Fixes #270

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/486>

nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero