Fix using right functions for file descriptor with MS Visual Studio

tls/compat/ftruncate.c

@@ -4,6 +4,8 @@
  * Kinichiro Inoguchi <inoguchi@openbsd.org>
  */
 
+#ifdef _WIN32
+
 #include <unistd.h>
 
 int
@@ -11,3 +13,5 @@ ftruncate(int fd, off_t length)
 {
 	return _chsize(fd, length);
 }
+
+#endif