More fixing MemoryBarrier() on Mingw32

Set requested windows header to Vista.  See discussion:

https://github.com/behdad/harfbuzz/commit/fbb2847f541389f40718af71c4945024ae177ab2#commitcomment-7054700

src/hb-atomic-private.hh

@@ -45,6 +45,9 @@
 #elif !defined(HB_NO_MT) && (defined(_WIN32) || defined(__CYGWIN__))
 
 #define WIN32_LEAN_AND_MEAN
+#ifndef _WIN32_WINNT
+#define _WIN32_WINNT 0x0600
+#endif
 #include <windows.h>
 
 /* MinGW has a convoluted history of supporting MemoryBarrier