icons: fix the native Windows build in CI with autotools.
Fixes:
make[3]: *** No rule to make target '64/dialog-question.png', needed by 'gimp-core-pixbufs.gresource.xml'. Stop.
It looks like a bug in autotools on Windows because there is no reason why it would fail. The missing PNG should be caught by the '64/%.png' rule. Anyway after testing, it fixes the gimp-win(32|64)-native CI jobs.