FTBFS under impish

[fossfreedom]

With the near release version of the impish packages, a sbuild & a ppa build of the latest impish plank sources no longer build

The last part of the sbuild output is:

Unity.vala:107.3-107.32: warning: Method `Plank.Unity.release_unity_dbus' never used
Command-line option `--driver` is deprecated and will be ignored
Succeeded - 84 warning(s)
touch plank_vala_doc.stamp
make all-am
make[4]: Entering directory '/<<PKGBUILDDIR>>/docs'
make[4]: *** No rule to make target 'vala-doc/plank/*.png', needed by 'all-am'. Stop.
make[4]: Leaving directory '/<<PKGBUILDDIR>>/docs'
make[3]: *** [Makefile:470: all] Error 2
make[3]: Leaving directory '/<<PKGBUILDDIR>>/docs'
make[2]: *** [Makefile:531: all-recursive] Error 1
make[2]: Leaving directory '/<<PKGBUILDDIR>>'
make[1]: *** [Makefile:462: all] Error 2
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
dh_auto_build: error: make -j16 returned exit code 2
make: *** [debian/rules:30: binary] Error 25
dpkg-buildpackage: error: debian/rules binary subprocess returned exit status 2
--------------------------------------------------------------------------------
Build finished at 2021-10-08T20:50:41Z

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: plank 0.11.89-3
ProcVersionSignature: Ubuntu 5.13.0-16.16-generic 5.13.13
Uname: Linux 5.13.0-16-generic x86_64
ApportVersion: 2.20.11-0ubuntu70
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: Budgie:GNOME
Date: Fri Oct 8 21:47:26 2021
InstallationDate: Installed on 2021-07-24 (76 days ago)
InstallationMedia: Ubuntu-Budgie 21.10 "Impish Indri" - Alpha amd64 (20210724)
SourcePackage: plank
SuspiciousXErrors.txt:

UpgradeStatus: No upgrade log present (probably fresh install)