Assistant not built in static Qt builds due to incorrect FEATURE_sql_sqlite condition

See https://codereview.qt-project.org/c/qt/qttools/+/304209/6/src/CMakeLists.txt#24

The assistant subdirectory is not added if FEATURE_sql_sqlite is false and Qt is a static build.

The FEATURE_sql_sqlite variable is never set in a per-repo build, because the feature is not exported on the Sql target. The sqldrivers/configure.cmake file is never associated to the Sql target, and thus none of the sqldrivers features are ever recorded / exported.
Presumably this was on purpose, because the sqldrivers folder is meant to build as a standalone project as well.

The sql plugin features are not exported in qmake Qt 5.15 builds as well.

We should find what's the appropriate CMake condition for qmake's 'contains(QT_PLUGINS, qtsqlite) , because checking for the feature currently worn't work.