mingw: spice-widget: make sure MAPVK_VK_TO_VSC is defined
mingw: spice-widget: make sure MAPVK_VK_TO_VSC is defined

MAPVK_VK_TO_VSC is defined in the file:
    <path-to-mingw-sys-root>/mingw/include/winuser.h

In older mingw-headers the definition of MAPVK_VK_TO_VSC is
defined only -- #if _WIN32_WINNT >= 0x0601

SPICE/spice-gtk,elmarco/spice-gtk,flexVDI/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk