BUILD: listener: add fcntl.h and unistd.h

Otherwise it fails to build on some platforms.
diff --git a/src/listener.c b/src/listener.c
index 05bb743..7ab1a87 100644
--- a/src/listener.c
+++ b/src/listener.c
@@ -14,6 +14,8 @@
 #include <errno.h>
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
 
 #include <common/accept4.h>
 #include <common/config.h>