OS X: Add retina / hidpi support to Info.plist.

This should partially fix the problem described at
https://ask.wireshark.org/questions/37177/pixelated-ui-on-mac-os-x-with-development-1990

Change-Id: I436aae0d5b681e9ff549d257c2244ee67448ab07
Reviewed-on: https://code.wireshark.org/review/4876
Reviewed-by: Gerald Combs <gerald@wireshark.org>