build: provide ssize_t definition for native Windows builds

- defines ssize_t in ice_socket.h and utf8_utils.cpp for
  Microsofts compiler.

Change-Id: I46131f613a35cabae9df7749221ba2f30d7fd888
Tuleap: #790

src/ice_socket.h

@@ -23,6 +23,11 @@
 #include <memory>
 #include <functional>
 
+#if defined(_MSC_VER)
+#include <BaseTsd.h>
+using ssize_t = SSIZE_T;
+#endif
+
 namespace ring {
 
 class IceTransport;

src/utf8_utils.cpp

@@ -25,6 +25,11 @@
 #include <cassert>
 #include "utf8_utils.h"
 
+#if defined(_MSC_VER)
+#include <BaseTsd.h>
+using ssize_t = SSIZE_T;
+#endif
+
 /*
  * The LIKELY and UNLIKELY macros let the programmer give hints to
  * the compiler about the expected result of an expression. Some compilers