Update assignment client to close on WM_CLOSE message on Windows

StackManagerQt sends a WM_CLOSE when closing a process, but by default they are not handled - this adds handling for those messages.
Based on the console output on the broken linux build I think this line is it the culprit.  The log says:
```
In file included from /mnt/ebs1/jenkins/jobs/hifi-pull-request-ubuntu/workspace/assignment-client/src/main.cpp:16:0:
/mnt/ebs1/jenkins/jobs/hifi-pull-request-ubuntu/workspace/assignment-client/src/AssignmentClient.h:16:40: fatal error: QAbstractNativeEventFilter.h: No such file or directory
```

Try changing "QAbstractNativeEventFilter.h" to "QAbstractNativeEventFilter" to see if that fixes it.