build: Fix vala build with gstapp in non-standard location

Some of the headers include <gst/app/gstappsink.h>, which is part of gstreamer-app-1.0, the pkg-config file only lists gstreamer-1.0 as a dependency, though. It is not guaranteed that they share the installation paths and indeed it is not the case on NixOS, resulting in build failure.

Let's properly declare our dependencies so that vala-gen-introspect can find them.

Fixes: #11 (closed)