Fix a header problem on Windows.

This commit was SVN r23483.

opal/util/fd.c

@@ -11,7 +11,9 @@
 
 #include "opal_config.h"
 
+#ifdef HAVE_UNISTD_H
 #include <unistd.h>
+#endif
 #include <errno.h>
 
 #include "opal/util/fd.h"