Need typedef of ssize_t for msc compiler

mrbgems/mruby-io/src/io.c

@@ -18,6 +18,7 @@
 #if defined(_WIN32) || defined(_WIN64)
   #include <winsock.h>
   #include <io.h>
+  #include <BaseTsd.h>
   #define open  _open
   #define close _close
   #define dup _dup
@@ -32,6 +33,7 @@
   typedef long fsuseconds_t;
   typedef int fmode_t;
   typedef int mrb_io_read_write_size;
+  typedef SSIZE_T ssize_t;
 
   #ifndef O_TMPFILE
     #define O_TMPFILE O_TEMPORARY