Debugger: Hide the reverse debugging feature

It's still available with QTC_DEBUGGER_ENABLE_REVERSE=1, but
since it rarely ever does something useful, it's better not
exposed in the gui.

Change-Id: If571fcfd01579c53f792d799027dd9d9a93c1cf4
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/debuggercore.h

@@ -112,6 +112,7 @@ QMessageBox *showMessageBox(int icon, const QString &title,
 
 bool isNativeMixedActive();
 bool isNativeMixedEnabled();
+bool isReverseDebuggingEnabled();
 
 } // namespace Internal
 } // namespace Debugger