Bump WINNT version requested from 500 to 600

Since we use the OpenType versions of Uniscribe functions, we are
relying on that version of the WINNT API.  Otherwise, usp10.h will hide
those symbols.

src/hb-uniscribe.cc

@@ -24,7 +24,7 @@
  * Google Author(s): Behdad Esfahbod
  */
 
-#define _WIN32_WINNT 0x0500
+#define _WIN32_WINNT 0x0600
 
 #include "hb-private.hh"
 

src/hb-uniscribe.h

@@ -29,7 +29,7 @@
 
 #include "hb.h"
 
-#define _WIN32_WINNT 0x0500
+#define _WIN32_WINNT 0x0600
 #include <windows.h>
 
 HB_BEGIN_DECLS