typedef: make ssize_t get typedef without LIBSSH2_WIN32

The condition around the ssize_t typedef depended on both LIBSSH2_WIN32
*and* _MSC_VER being defined when it should be enough to depend on
_MSC_VER only. It also makes it nicer so libssh2-using code builds fine
without having custom defines.

include/libssh2.h

@@ -73,7 +73,7 @@ typedef unsigned char uint8_t;
 typedef unsigned int uint32_t;
 #endif
 
-#if defined(LIBSSH2_WIN32) && defined(_MSC_VER)
+#ifdef _MSC_VER
 typedef unsigned char uint8_t;
 typedef unsigned int uint32_t;
 typedef unsigned __int64 libssh2_uint64_t;