matroskademux: Unmap wavpack header buffer after creating it
matroskademux: Unmap wavpack header buffer after creating it

Otherwise it will be mapped writable all the time and we can't read from it
anywhere.

https://bugzilla.gnome.org/show_bug.cgi?id=762239

hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good