ast: Fix warning in sprintf-style ErsRep call

Using a variable rather than a constant string in a printf-style
argument now seems to be an error in clang rather than simply
a warning. Use "%s" instead.
(cherry picked from commit 3b669f3)

libraries/ast/err_drama.c

@@ -118,5 +118,5 @@ void astPutErr_( int status, const char *message ) {
    report the error message through EMS and to associate the error
    status with it. Ignore any returned status value. */
    local_status = status;
-   ErsRep( 0, &local_status, message );
+   ErsRep( 0, &local_status, "%s", message );
 }