doc: clangqdoc doesn't need Q_GADGET

This update undefines Q_GADGET when clangqdoc is running.
With Q_GADGET defined, clangqdoc expects documentation for
QtGadgetHelper in each class where Q_GADGET is used.

Note that the documentation for the Q_GADGET macro is still
generated.

Change-Id: I3e6e6644c2c537c00c44ae77664892e125046daa
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/corelib/kernel/qobjectdefs.h

@@ -240,6 +240,11 @@ private: \
 #define Q_SLOT Q_SLOT
 #endif //Q_MOC_RUN
 
+#ifdef Q_CLANG_QDOC
+#undef Q_GADGET
+#define Q_GADGET
+#endif
+
 #ifndef QT_NO_META_MACROS
 // macro for onaming members
 #ifdef METHOD