Build winshutdownmonitor.cpp on Windows only.

The file ``qt/winshutdownmonitor.cpp`` is for Windows only. It is marked as such inside (completely #ifdef Windows). But it is compiled and linked in even on non-Windows OS. This change marks it Windows only completely and thus prevents linker warning on newer systems:
```
/usr/bin/ranlib: file: qt/libbitcoinqt.a(qt_libbitcoinqt_a-winshutdownmonitor.o) has no symbols
```
What can be done more: the file ``bitcoind.cpp`` contains: ``#include "winshutdownmonitor.h"`` - this can be also #ifdefed, but as the included header file is itself #ifdef Windows only, it does not have any effect.