wfreerdp: kbd: fix ctrl/alt syncronization.

Fix the following issue:
In the remote desktop window, press the ctrl key down, hold on, and click a local window to make the focus to leave the remote desktop window, the ctrl key-up message will be lost for the remote desktop. It will recognize the ctrl key as down when the focus goes back to the remote desktop.
It is the same when you press the key down in the local window, and release it in the remote window.