libssh2.h: Fix Error C2371 'ssize_t': redefinition

Closes #331

include/libssh2.h

@@ -137,8 +137,9 @@ typedef __int64 int64_t;
 typedef unsigned __int64 uint64_t;
 typedef unsigned __int64 libssh2_uint64_t;
 typedef __int64 libssh2_int64_t;
-#ifndef ssize_t
+#if (!defined(HAVE_SSIZE_T) && !defined(ssize_t))
 typedef SSIZE_T ssize_t;
+#define HAVE_SSIZE_T
 #endif
 #else
 #include <stdint.h>