multimedia/gstreamer1-plugins: Introduce GST_PLUGIN_DEPENDS

Some plugins such as graphics/gstreamer1-plugins-vulkan has a dependency
of wayland which previously were handled by passing both CONFIGURE_ARGS and
MESON_ARGS via the gstreamer1 "framework" as the framework isn't
aware if dependencies to get what would eventually end up in MESON_ARGS in
correct order. This approach broke with commit
bf193dc1da282c769d3291817a8378adbd762ac7 as we now only make use of
MESON_ARGS instead of incorrectly using CONFIGURE_ARGS to pass
MESON_ARGS. In order to fix this introduce GST_PLUGIN_DEPENDS which
gets appended after gstreamer's current framework logic in MESON_ARGS.
It only supports defining one dependency but it's sufficient fow now.