Use a private stuct to not break ABI.

Use a private stuct to not break ABI.

Original commit message from CVS:
* docs/libs/gstreamer-libs-sections.txt:
* libs/gst/net/gstnetclientclock.c:
(gst_net_client_clock_class_init), (gst_net_client_clock_init),
(gst_net_client_clock_finalize), (gst_net_client_clock_do_select),
(gst_net_client_clock_thread), (gst_net_client_clock_start),
(gst_net_client_clock_stop), (gst_net_client_clock_new):
* libs/gst/net/gstnetclientclock.h:
* libs/gst/net/gstnettimeprovider.c:
(gst_net_time_provider_class_init), (gst_net_time_provider_init),
(gst_net_time_provider_finalize), (gst_net_time_provider_thread),
(gst_net_time_provider_start), (gst_net_time_provider_stop),
(gst_net_time_provider_new):
* libs/gst/net/gstnettimeprovider.h:
Use a private stuct to not break ABI.