build: add <io.h> to the includes within MSVC/unistd.h

- adds <io.h> to the Windows unistd.h substitute, providing the
  definition for access().

Change-Id: Ib054a1608e40f6fe22c0c277ba204d1b5e506940
Tuleap: #790

MSVC/unistd.h

@@ -26,6 +26,7 @@
 #include <process.h>
 #include <direct.h>
 #include <fcntl.h>
+#include <io.h>
 
 #define R_OK    4
 #define W_OK    2