Added include protection for unistd.h and sys/time.h.

diff --git a/files/dos_fs.c b/files/dos_fs.c
index da71ae2..3b0abea2 100644
--- a/files/dos_fs.c
+++ b/files/dos_fs.c
@@ -36,7 +36,9 @@
 #include <sys/ioctl.h>
 #endif
 #include <time.h>
-#include <unistd.h>
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>
+#endif
 
 #include "windef.h"
 #include "winerror.h"