wpe: initialize threading.ready before reading it

Fix this Valgrind warning:

 Thread 5 wpevideosrc0:src:
 Conditional jump or move depends on uninitialised value(s)
    at 0x49C0750: GstWPEContextThread::GstWPEContextThread() (gstwpethreadedview.cpp:80)
    by 0x49C0804: GstWPEContextThread::singleton() (gstwpethreadedview.cpp:64)
    by 0x49C2DA7: gst_wpe_video_src_start(_GstWpeVideoSrc*) (gstwpevideosrc.cpp:298)
    by 0x130F33DC: gst_base_src_prepare_allocation (gstbasesrc.c:3373)
    by 0x130F33DC: gst_base_src_negotiate_unlocked (gstbasesrc.c:3511)
    by 0x130F5717: gst_base_src_loop (gstbasesrc.c:2920)
    by 0x4921C41: gst_task_func (gsttask.c:399)
    by 0x4A5D541: g_thread_pool_thread_proxy.lto_priv.0 (gthreadpool.c:336)
    by 0x4A5B812: g_thread_proxy (gthread.c:835)
    by 0x4C126D6: start_thread (pthread_create.c:447)
    by 0x4C96413: clone (clone.S:100)