package/gstreamer[0/1]: rename BR2_PACKAGE_GST_PLUGINS_GOOD_PLUGIN_QTDEMUX config option.

 1] BR2_PACKAGE_GST_PLUGINS_GOOD_PLUGIN_QTDEMUX is not defined anywhere
 2] qtdemux is not a plugin by itself but belongs to isomp4

Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>