xz: Make args_info.files_name a const pointer.

src/xz/args.c

@@ -490,7 +490,7 @@ parse_real(args_info *args, int argc, char **argv)
 						"or `--files0'."));
 
 			if (optarg == NULL) {
-				args->files_name = (char *)stdin_filename;
+				args->files_name = stdin_filename;
 				args->files_file = stdin;
 			} else {
 				args->files_name = optarg;

src/xz/args.h

@@ -19,7 +19,7 @@ typedef struct {
 
 	/// Name of the file from which to read filenames. This is NULL
 	/// if --files or --files0 was not used.
-	char *files_name;
+	const char *files_name;
 
 	/// File opened for reading from which filenames are read. This is
 	/// non-NULL only if files_name is non-NULL.