Keep gcc -Wall happy.
diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index c53adc1..3cbd4e8 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -39,6 +39,10 @@
 #include "modsupport.h"
 #include "ceval.h"
 
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
 #include <sys/types.h>
 
 #ifdef MS_WINDOWS