(main): Cast NULL to `(char *)' in

call to variadic version_etc function, so that it works even on systems
for which sizeof char* != sizeof int.

src/true.c

@@ -63,7 +63,8 @@ main (int argc, char **argv)
 	usage (EXIT_SUCCESS);
 
       if (STREQ (argv[1], "--version"))
-	version_etc (stdout, PROGRAM_NAME, GNU_PACKAGE, VERSION, AUTHORS, NULL);
+	version_etc (stdout, PROGRAM_NAME, GNU_PACKAGE, VERSION, AUTHORS,
+		     (char *) NULL);
     }
 
   exit (EXIT_SUCCESS);