Fix \since for QDrag::cancel()

This was added in 5.7.0 alpha, not 5.6.

Change-Id: Id8c68eca806274c77e7fa6904f21224fb468c8e4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/kernel/qdrag.cpp

@@ -393,7 +393,7 @@ Qt::DropAction QDrag::defaultAction() const
 
     \note This is currently implemented on Windows and X11.
 
-    \since 5.6
+    \since 5.7
     \sa exec()
 */
 void QDrag::cancel()

src/gui/kernel/qplatformdrag.cpp

@@ -166,7 +166,7 @@ Qt::DropAction QPlatformDrag::defaultAction(Qt::DropActions possibleActions,
 
     The default implementation does nothing.
 
-    \since 5.6
+    \since 5.7
  */
 
 void QPlatformDrag::cancelDrag()