gstmessage.h: Avoid gcc bit shift overflow compiler warning
gstmessage.h: Avoid gcc bit shift overflow compiler warning

Avoids bit shift overflow warning with gcc6.

https://bugzilla.gnome.org/show_bug.cgi?id=767882 (glib)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71803 (gcc)
https://bugzilla.gnome.org/show_bug.cgi?id=767883

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer