Add const qualifier to tzn returned by timestamp2tm()

The tzn value might come from tm->tm_zone, which libc declares as
const, so it's prudent that the upper layers know about this as well.

src/backend/utils/adt/formatting.c

@@ -459,7 +459,7 @@ typedef struct TmToChar
 {
 	struct pg_tm tm;			/* classic 'tm' struct */
 	fsec_t		fsec;			/* fractional seconds */
-	char	   *tzn;			/* timezone */
+	const char *tzn;			/* timezone */
 } TmToChar;
 
 #define tmtcTm(_X)	(&(_X)->tm)