qttools 5.15 fails to compile with a linker error

It seems to be caused by commit b3cdd63d4bdaea09222fb93ffcd5104a2dc0bf2e

Reverting the commit allows it to build again, otherwise I get

```

g++ Wl,no-undefined -Wl,-O1 -Wl,-enable-new-dtags -Wl,-z,origin -Wl,-rpath,\$ORIGIN/../../lib/x86_64-linux-gnu -shared -o libqquickwidget.so .obj/qquickwidget_plugin.o .obj/qrc_qquickwidget_plugin.o .obj/moc_qquickwidget_plugin.o /srcb$
mv -f libqwebview.so ../../../../../plugins/designer/libqwebview.so
make[5]: Leaving directory '/srcbuild/qt5-qttools/src/designer/src/plugins/qwebview'
g++ -c -include .pch/Qt5DesignerComponents -pipe -Os -std=c++1z -fvisibility=hidden -fvisibility-inlines-hidden -fno-exceptions -Wall -Wextra -Wvla -Wdate-time -Wshift-overflow=2 -Wduplicated-cond -Wno-stringop-overflow -D_REENTRANT -fPIC -$
/usr/bin/ld: .obj/moc_helpviewer.o.data.rel.ro._ZTV10HelpViewer[_ZTV10HelpViewer]+0x110): undefined reference to `HelpViewer::resizeEvent(QResizeEvent*)'
collect2: error: ld returned 1 exit status
make[3]: *** [Makefile:274: ../../../bin/assistant] Error 1
make[3]: Leaving directory '/srcbuild/qt5-qttools/src/assistant/assistant'
make[2]: *** [Makefile:75: sub-assistant-make_first] Error 2
make[2]: Leaving directory '/srcbuild/qt5-qttools/src/assistant'
make[1]: *** [Makefile:57: sub-assistant-make_first] Error 2
make[1]: *** Waiting for unfinished jobs....

```