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/os_windows.cpp

@@ -677,7 +677,7 @@ LRESULT OS_Windows::WndProc(HWND hWnd,UINT uMsg, WPARAM	wParam,	LPARAM	lParam) {
 			print_line("input lang change");
 		} break;
 
-		#if WINVER >= 0x0700 // for windows 7
+		#if WINVER >= 0x0601 // for windows 7
 		case WM_TOUCH: {
 
 			BOOL bHandled = FALSE;