Missing gstreamer dependency

[Michi Henning]

We use ubuntu-app-platform for the thumbnailer. The thumbnailer adds gstreamer1.0-libav for H.264 video. When thumbnailing an mp4 file, we get the following warning on xenial:

(gst-plugin-scanner:28494): GStreamer-WARNING **: Failed to load plugin '/snap/thumbnailer/x1/ubuntu-app-platform/usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgstaasink.so': libgpm.so.2: cannot open shared object file: No such file or directory

It looks like ubuntu-app-platform installs libgstaasink.so, but doesn't install libgpm2. That package contains libgpm.so.2, which is loaded by libstaasink.so.