bufferpool: unblock acquire thread when we discard buffer

else the acquire thread will wait infinitely.
The deadlock showed in prevous unit test commit. This will fix it

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/470>

gst/gstbufferpool.c

@@ -1326,6 +1326,7 @@ not_writable:
 discard:
   {
     do_free_buffer (pool, buffer);
+    gst_poll_write_control (pool->priv->poll);
     return;
   }
 }