Itroduce XLAT_END macro to make xlat structures more compact

* defs.h (XLAT_END): New macro.
diff --git a/defs.h b/defs.h
index 8213ea4..b0a9144 100644
--- a/defs.h
+++ b/defs.h
@@ -504,6 +504,7 @@
 	const char *str;
 };
 #define XLAT(x) { x, #x }
+#define XLAT_END { 0, NULL }
 
 extern const struct xlat open_mode_flags[];
 extern const struct xlat addrfams[];