msvc: fix build

Change-Id: Ia26ef6c94b52efa929e974384f6f8258126453ae
diff --git a/include/ice_transport.h b/include/ice_transport.h
index c5047b3..8c69e93 100644
--- a/include/ice_transport.h
+++ b/include/ice_transport.h
@@ -26,6 +26,11 @@
 #include <vector>
 #include <chrono>
 
+#if defined(_MSC_VER)
+#include <BaseTsd.h>
+using ssize_t = SSIZE_T;
+#endif
+
 extern "C" {
 struct pj_ice_sess_cand;
 }