The typedef is only present in the .sip file, but without this
annotation SIP assumes it also exists in QGIS headers. This didn't
completely break the build, since we concatenate multiple SIP headers
into larger files, so the typedefs from one part "fixed" another. Run
sip-build without --concatenate to see the issue clearly.
This fix is currently sadly not fully effective due to a SIP bug:
https://github.com/Python-SIP/sip/issues/66
4d86676c35