Debugger: Support gdbservers that don't have --once again

There are too many setup without support for --once.

Change-Id: I172b8b8d58a3960d9d93b48e692e612b7627ba56
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Tim Sander <tim@krieglstein.org>

src/plugins/debugger/gdb/gdbengine.cpp

@@ -1945,6 +1945,7 @@ void GdbEngine::notifyAdapterShutdownOk()
     m_commandsDoneCallback = 0;
     switch (m_gdbProc->state()) {
     case QProcess::Running:
+        postCommand("monitor exit");
         postCommand("-gdb-exit", GdbEngine::ExitRequest, CB(handleGdbExit));
         break;
     case QProcess::NotRunning: