zbar: add JPEG-related patch

This patch was applied upstream, but has not yet made it into any official release. Without this patch, `libjpeg` is unable to seek within a JPEG datastream, resulting in hard-to-diagnose failures.

This patch originates from [this Sourceforge discussion](http://sourceforge.net/p/zbar/discussion/664596/thread/58b8d79b) (also linked in the updated formula) and fixes willglynn/ruby-zbar#2.