LinGui: fix live preview generation

Preferences (where live preview duration is set) were not getting added
to live peview job settings.

Also, fix a compiler warning when using gstreamer 0.10.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6127 b64f7644-9d1e-0410-96f1-a4d463321fa5