Always #define INVALID_SOCKET -1 in libssh2_priv.h when not on win32

Fix broken builds since commit abd9bd0bbe631efeada1f54552c70b54e1c490c1
for all non-win32 platforms.

src/agent.c

@@ -124,10 +124,6 @@ struct agent_ops {
     agent_disconnect_func disconnect;
 };
 
-#ifndef INVALID_SOCKET
-#define INVALID_SOCKET -1
-#endif
-
 struct _LIBSSH2_AGENT
 {
     LIBSSH2_SESSION *session;  /* the session this "belongs to" */

src/libssh2_priv.h

@@ -154,6 +154,7 @@ static inline int writev(int sock, struct iovec *iov, int nvecs)
 typedef SOCKET libssh2_socket_t;
 #else /* !WIN32 */
 typedef int libssh2_socket_t;
+#define INVALID_SOCKET -1
 #endif /* WIN32 */
 
 /* RFC4253 section 6.1 Maximum Packet Length says: