QtQuick not included when cross-compiling in a top-level build due to missing qsb

1The change in https://codereview.qt-project.org/c/qt/qtdeclarative/+/348291 introduces a "TARGET Qt::qsb" - CMake condition for including QtQuick into the build.

When cross-compiling this target is not set and QtQuick is not included as a consequence.
After initial analysis with Joerg it seems the versionless "Qt::qsb" is not promoted to global scope in qtshadertools, whereas the Qt6::qsb is defined.

Temporary workaround is to change "Qt::qsb" => "Qt6::qsb" in qtdeclarative/src/CMakeLists.txt