Add missing #include.

--- compface-1.5.2/config.h	2005-10-04 13:29:00.000000000 +0100
+++ compface-1.5.2/config.h	2025-07-03 16:51:11.127114049 +0100
@@ -30,3 +30,4 @@
 #endif
 
 #include <stdio.h>
+#include <unistd.h>