message: [Build] compile/link winshutdownmonitor.cpp on Windows only
  The file `qt/winshutdownmonitor.cpp` is for Windows only. Excluding it
  on other systems prevents a linker warning about missing symbols at
  compile time.
diff: @@ -271,8 +271,11 @@ BITCOIN_QT_CPP = \
  qt/rpcconsole.cpp \
  qt/splashscreen.cpp \
  qt/trafficgraphwidget.cpp \
  - qt/utilitydialog.cpp \
  - qt/winshutdownmonitor.cpp
  + qt/utilitydialog.cpp
  +
  +if TARGET_WINDOWS
  +BITCOIN_QT_CPP += qt/winshutdownmonitor.cpp
  +endif
  if ENABLE_WALLET
  BITCOIN_QT_CPP += \
  