Implement glib::SendUnique on Adapter

It can be safely sent between threads as long as all references are in
the same thread.

Gir_GstBase.toml

@@ -39,7 +39,7 @@ manual = [
 name = "GstBase.Adapter"
 status = "generate"
 trait = false
-concurrency = "none"
+concurrency = "send-unique"
 
     [[object.function]]
     name = "map"

gstreamer-base/src/auto/adapter.rs

@@ -4,6 +4,7 @@
 
 use ffi;
 use glib;
+use glib::ObjectExt;
 use glib::translate::*;
 use glib_ffi;
 use gobject_ffi;
@@ -193,3 +194,9 @@ impl Default for Adapter {
         Self::new()
     }
 }
+
+unsafe impl glib::SendUnique for Adapter {
+    fn is_unique(&self) -> bool {
+        self.ref_count() == 1
+    }
+}