examples/onvif-server: fix werror build with clang

examples/onvif-server: fix werror build with clang

../subprojects/gst-rtsp-server/examples/test-onvif-server.c:346:65: warning: implicit conversion from enumeration type 'const GstSegmentFlags' to different enumeration type 'GstSeekFlags' [-Wenum-conversion]
        self->incoming_segment->format, self->incoming_segment->flags,
                                        ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
../subprojects/gst-rtsp-server/examples/test-onvif-server.c:53:1: warning: unused function 'REPLAY_IS_BIN' [-Wunused-function]
G_DECLARE_FINAL_TYPE (ReplayBin, replay_bin, REPLAY, BIN, GstBin);
^
/usr/include/glib-2.0/gobject/gtype.h:1407:26: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
  static inline gboolean MODULE##_IS_##OBJ_NAME (gpointer ptr) {                                         \
                         ^
<scratch space>:77:1: note: expanded from here
REPLAY_IS_BIN
^
../subprojects/gst-rtsp-server/examples/test-onvif-server.c:525:1: warning: unused function 'ONVIF_FACTORY' [-Wunused-function]
G_DECLARE_FINAL_TYPE (OnvifFactory, onvif_factory, ONVIF, FACTORY,
^
/usr/include/glib-2.0/gobject/gtype.h:1405:33: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
  static inline ModuleObjName * MODULE##_##OBJ_NAME (gpointer ptr) {                                     \
                                ^
<scratch space>:9:1: note: expanded from here
ONVIF_FACTORY
^
../subprojects/gst-rtsp-server/examples/test-onvif-server.c:525:1: warning: unused function 'ONVIF_IS_FACTORY' [-Wunused-function]
/usr/include/glib-2.0/gobject/gtype.h:1407:26: note: expanded from macro 'G_DECLARE_FINAL_TYPE'
  static inline gboolean MODULE##_IS_##OBJ_NAME (gpointer ptr) {                                         \
                         ^
<scratch space>:12:1: note: expanded from here
ONVIF_IS_FACTORY
^