debian/control: add to B-Ds: libqt5multimedia5-plugins

This seems to fix test failure due to the test racing with an error dialog due to missing plugin, causing unexpected state to the tests.

I'm still not sure why it's a problem only on armhf but not other architectures, but, I mean, if you already include gstreamer1.0- plugins-base, then not having a plugin to use it is silly.

Fixes: #19 (closed)