Bug#1071436: qt6: mixing incompatible versions is not precluded

[Oswald Buddenhagen <oswald.buddenhagen@gmx.de>]

Package: qt6-5compat-dev
Version: 6.6.2-1+b1
Severity: normal

i installed qt6-l10n-tools 6.6 from experimental, which also upgraded
qtbase and some other packages. however, it left qt6-5compat-dev at the
unstable 6.4, thus breaking subsequent builds due to binary
incompatibility; i got a bunch of errors looking like

  /usr/bin/ld: /usr/lib/x86_64-linux-gnu/libQt6Core5Compat.so.6.4.2: undefined reference to `QUtf16::convertToUnicode(QByteArrayView, QStringConverterBase::State*, DataEndianness)@Qt_6'


manually pulling qt6-5compat-dev from experimental as well fixed the
problem.