\SRF\Gallery jCarousel: No width/height set for items

Add jcarousel/itemFallbackDimension according to [1]

[1] http://stackoverflow.com/questions/3784925/no-width-height-set-for-items-this-will-cause-an-infinite-loop-aborting

Change-Id: I320402cb616339d937fcbae6825cb738319a5f29