Fix build when -Werror=deprecated-declarations is on

Fix build when -Werror=deprecated-declarations is on

As gst_rtsp_session_next_timeout is deprecated.

```
../subprojects/gst-rtsp-server/gst/rtsp-server/rtsp-session.c:760:3: error: ‘gst_rtsp_session_next_timeout’ is deprecated: Use 'gst_rtsp_session_next_timeout_usec' instead [-Werror=deprecated-declarations]
   res = (gst_rtsp_session_next_timeout (session, now) == 0);
   ^~~
../subprojects/gst-rtsp-server/gst/rtsp-server/rtsp-session.c:685:1: note: declared here
 gst_rtsp_session_next_timeout (GstRTSPSession * session, GTimeVal * now)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```