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

Index: libweston/renderer-gl/gl-renderer.c
--- libweston/renderer-gl/gl-renderer.c.orig
+++ libweston/renderer-gl/gl-renderer.c
@@ -71,6 +71,8 @@
 #include "shared/xalloc.h"
 #include "tests/harness/weston-testsuite-data.h"
 
+#include "shared/os-compatibility.h"
+
 #define BUFFER_DAMAGE_COUNT 2
 
 #define SWIZZLES_AG00 { GL_ALPHA, GL_GREEN, GL_ZERO, GL_ZERO }
