CMake: Add public dependency from Qt::Dbus to Qt::Core

Anything that uses Qt::Dbus also needs to link to Qt::Core, so encode
that in the target.

Change-Id: Ibc984d5b5e118aae8d1c0d21117fcff1664b0ae3
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/dbus/CMakeLists.txt

@@ -43,6 +43,8 @@ add_qt_module(DBus
         QT_NO_FOREACH
     LIBRARIES
         Qt::CorePrivate
+    PUBLIC_LIBRARIES
+        Qt::Core # special case
     # MODULE_CONFIG = "dbusadaptors" "dbusinterfaces"
     # _LOADED = "qt_module"
 )