OpenBSD does not pull in os-compatibility.h transitively; include it explicitly.

Index: shared/config-parser.c
--- shared/config-parser.c.orig
+++ shared/config-parser.c
@@ -44,6 +44,7 @@
 #include <libweston/libweston.h>
 #include "helpers.h"
 #include "string-helpers.h"
+#include "os-compatibility.h"
 
 /**
  * \defgroup weston-config Weston configuration
