Bug#945545: xpp FTBFS: error: invalid use of incomplete type 'ipp_t'

[Helmut Grohne <helmut@subdivi.de>]

Source: xpp
Version: 1.5-cvs20081009-4
Severity: serious
Tags: ftbfs

xpp fails to build from source in unstable.

https://tests.reproducible-builds.org/debian/rbuild/unstable/amd64/xpp_1.5-cvs20081009-4.rbuild.log.gz
| cupshelper.cxx: In static member function 'static ipp_t* CupsHelper::newIppRequest()':
| cupshelper.cxx:97:10: error: invalid use of incomplete type 'ipp_t' {aka 'struct _ipp_s'}
|    97 |   request->request.op.request_id = 1;
|       |          ^~
| In file included from /usr/include/cups/cups.h:27,
|                  from cupshelper.h:36,
|                  from cupshelper.cxx:35:
| /usr/include/cups/ipp.h:731:16: note: forward declaration of 'ipp_t' {aka 'struct _ipp_s'}
|   731 | typedef struct _ipp_s ipp_t;  /**** IPP request/response data ****/
|       |                ^~~~~~

Also seen by crossqa since about two months:
http://crossqa.subdivi.de/build/xpp_1.5-cvs20081009-4_armel_20190914094237.log

Helmut