Fix Yosemite build

From CUPS 1.6, the visibility of parts of the IPP data structures controlled are by the preprocessor symbol IPP_PRIVATE_STRUCTURES.
Visibility is enabled specifically for Mac OS X 10.8 and 10.9 but not for Mac OS X 10.10 in CUPS. This update forces the required visibility.
Merge pull request #18 from davidbernard/yosemite

Fix Yosemite build