OpenBSD does not pull in <sys/stat.h> transitively; include it explicitly.

Index: libweston/backend-x11/x11.c
--- libweston/backend-x11/x11.c.orig
+++ libweston/backend-x11/x11.c
@@ -37,6 +37,7 @@
 #include <errno.h>
 #include <sys/time.h>
 #include <sys/shm.h>
+#include <sys/stat.h>
 #include <linux/input.h>
 
 #include <xcb/xcb.h>
