Windows: Define _WIN32_WINRT to 0x0600 (Vista)

Passed as a compiler define to be sure it is always define before windows.h
is loaded. This means that Godot officially requires Vista API or later, it will
not work on Windows XP or earlier.

Also fix a bogus check for Windows 7 API.

platform/windows/key_mapping_win.cpp

@@ -26,8 +26,8 @@
 /* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
-#define WINVER 0x0500
 #include "key_mapping_win.h"
+
 #include <stdio.h>
 
 struct _WinTranslatePair {