android release tarballs do not include .pc files for plugins

On android, plugins are linked statically in the application, so we need to have the plugin pkg-config files installed as well. These are normally placed in $prefix/lib/gstreamer-1.0/pkgconfig/

You can easily get those if you build from source, but the android tarballs in https://gstreamer.freedesktop.org/download/ do not include them.