sys/windows: add Dwmapi.lib binding

core/sys/windows/dwmapi.odin

@@ -0,0 +1,9 @@
+// +build windows
+package sys_windows
+
+foreign import dwmapi "system:Dwmapi.lib"
+
+@(default_calling_convention="stdcall")
+foreign dwmapi {
+	DwmFlush :: proc() -> HRESULT ---
+}