player: eventfd leak on GMainContext
valgrind unit test with
--track-fds=yes option reported that there is eventfd leak.
$GST_CHECKS=test_create_and_free make libs/player.valgrind ==10668== FILE DESCRIPTORS: 5 open at exit. ==10668== Open file descriptor 5: ==10668== at 0x5CA0A87: eventfd (syscall-template.S:78) ==10668== by 0x58FABA6: g_wakeup_new (gwakeup.c:146) ==10668== by 0x589F6AE: g_main_context_new (gmain.c:658) ==10668== by 0x589F755: g_main_context_default (gmain.c:694) ==10668== by 0x589F80B: g_main_context_push_thread_default (gmain.c:783) ==10668== by 0x4E4B9E2: gst_player_main (gstplayer.c:2882) ==10668== by 0x58D4197: g_thread_proxy (gthread.c:784) ==10668== by 0x6C726DA: start_thread (pthread_create.c:463) ==10668== by 0x5CA088E: clone (clone.S:95)