gstptpclock: Don't leak the GList

gstptpclock: Don't leak the GList

120 bytes in 5 blocks are definitely lost in loss record 7,615 of 9,510
   at 0x484486F: malloc (vg_replace_malloc.c:380)
   by 0x58A2938: g_malloc (gmem.c:106)
   by 0x58BA1F4: g_slice_alloc (gslice.c:1069)
   by 0x588F059: g_list_prepend (glist.c:335)
   by 0x5B9C5C0: select_best_master_clock (gstptpclock.c:756)
   by 0x5B9CA8E: cleanup_cb (gstptpclock.c:1930)
   by 0x589AD20: g_timeout_dispatch (gmain.c:4889)
   by 0x589A4CE: UnknownInlinedFun (gmain.c:3337)
   by 0x589A4CE: g_main_context_dispatch (gmain.c:4055)
   by 0x58EE4E7: g_main_context_iterate.constprop.0 (gmain.c:4131)
   by 0x5899A92: g_main_loop_run (gmain.c:4329)
   by 0x5B9BA4C: ptp_helper_main (gstptpclock.c:1980)
   by 0x58C8C31: g_thread_proxy (gthread.c:826)

576 bytes in 24 blocks are definitely lost in loss record 8,782 of 9,510
   at 0x484486F: malloc (vg_replace_malloc.c:380)
   by 0x58A2938: g_malloc (gmem.c:106)
   by 0x58BA1F4: g_slice_alloc (gslice.c:1069)
   by 0x588F059: g_list_prepend (glist.c:335)
   by 0x5B9C5C0: select_best_master_clock (gstptpclock.c:756)
   by 0x5B9EFA0: handle_announce_message (gstptpclock.c:934)
   by 0x5B9EFA0: handle_ptp_message (gstptpclock.c:1765)
   by 0x5B9EFA0: have_stdin_data_cb (gstptpclock.c:1851)
   by 0x589A4CE: UnknownInlinedFun (gmain.c:3337)
   by 0x589A4CE: g_main_context_dispatch (gmain.c:4055)
   by 0x58EE4E7: g_main_context_iterate.constprop.0 (gmain.c:4131)
   by 0x5899A92: g_main_loop_run (gmain.c:4329)
   by 0x5B9BA4C: ptp_helper_main (gstptpclock.c:1980)
   by 0x58C8C31: g_thread_proxy (gthread.c:826)
   by 0x5DA4298: start_thread (pthread_create.c:481)

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/852>