Doc: Add qmake include flag for module

Fixes: QTBUG-63834
Change-Id: I8ba52d68a878ec61337e8111d233409a9a98f47d
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

src/imports/controls/doc/src/qtquickcontrols2-cppclasses.qdoc

@@ -30,6 +30,8 @@
     \keyword Qt Quick Controls C++ Classes
     \title Qt Quick Controls 2 C++ Classes
     \ingroup modules
+    \qtvariable quickcontrols2
+
     \brief Provides classes for setting up the controls from C++.
 
     The C++ types can be included into your application using the following