fix: use accessors instead of direct access to now-private structures

[philomory]

Starting with CUPS 1.6, various structs such as ipp_attribute_t were made private, with the intent that their members be accessed via accessor functions such as ippNextAttribute() and ippGetGroupTag(). In CUPS 2.3.3, the _IPP_PRIVATE_STRUCTURES define was removed entirely, so the only means to read these structs became using the accessor methods.

This patch simply replaces direct member access with the use of the relevant accessor methods, so that the notifier can be compiled with for use with CUPS 2.3.3+.

[iliyanotlioneberia]

Hi @philomory and thank you for your interest and work to produce this PR! Unfortunately, the creator of this repo stopped maintaining it a while ago and I stopped using CUPS altogether even before that event occurred. Regardless I was looking at 2.3.3 documentation, patch notes and code commits but I couldn't find any information about dropping _IPP_PRIVATE_STRUCTURES define. Do you care to share any relevant information?

[philomory]

There wasn't a lot of fanfare about it, but you can see that it was removed in this commit in 2018: OpenPrinting/cups@0fb02fb

(So, as it turns out, this wasn't a change in 2.3.3, but actually in 2.3rc1)