Move QMetaMethod ctor back into a private section

Its symbol changed once it was changed to protected, so even if
it's unlikely to have been imported in any application let's just
change it back to private.

Pick-to: 6.5
Change-Id: I21b4a471caedb8edc8e1a04f706eae541b3c12fd
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qmetaobject.h

@@ -240,9 +240,11 @@ protected:
 
         const uint *d;
     };
+private:
     constexpr QMetaMethod(const QMetaObject *metaObject, const Data &data_)
         : mobj(metaObject), data(data_)
     {}
+protected:
 
     const QMetaObject *mobj;
     Data data;