Debugger: Force flushing of GDB output

... after continue and step. Otherwise there would be no reaction
after the inferior gets an external SIGKILL while stopped.

Task-number: QTCREATORBUG-17280
Change-Id: I57933e92194a3320d7aba81055755477f34af382
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/gdb/gdbengine.h

@@ -154,6 +154,8 @@ private: ////////// Gdb Command Management //////////
         NeedsStop = 1,
         // No need to wait for the reply before continuing inferior.
         Discardable = 2,
+        // Needs a dummy extra command to force GDB output flushing.
+        NeedsFlush = 4,
         // Callback expects ResultRunning instead of ResultDone.
         RunRequest = 16,
         // Callback expects ResultExit instead of ResultDone.