Introduce XLAT macro to ease maintenance of xlat structures

* defs.h (XLAT): New macro.

Suggested-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/defs.h b/defs.h
index 330441f..8213ea4 100644
--- a/defs.h
+++ b/defs.h
@@ -503,6 +503,7 @@
 	int val;
 	const char *str;
 };
+#define XLAT(x) { x, #x }
 
 extern const struct xlat open_mode_flags[];
 extern const struct xlat addrfams[];