fix background in dmg installer for osx
For the OSX dmg installer, the background image isn't getting displayed in the little window where the user drags Purr Data's icon into the Applications folder.
The relevant background image is purr-data/packages/darwin_app/background.png
Might want to check both purr-data/packages/darwin_app/Makefile as well as the build output to see if there's an error somewhere.