webrtc: Add missing gst dependency

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1382>

gstreamer-webrtc/sys/Gir.toml

@@ -19,6 +19,7 @@ external_libraries = [
 ]
 
 [external_libraries]
+gstreamer = "Gst"
 gstreamer_sdp = "GstSdp"
 
 [[object]]

gstreamer-webrtc/sys/src/lib.rs

@@ -14,6 +14,7 @@
 
 use glib_sys as glib;
 use gstreamer_sdp_sys as gst_sdp;
+use gstreamer_sys as gst;
 
 #[allow(unused_imports)]
 use libc::{