Use function format attribute where applicable

Allow the compiler to verify the format string against the supplied
arguments.

    chage.c:239:51: warning: format not a string literal, format string not checked [-Wformat-nonliteral]
      239 |                 (void) strftime (buf, sizeof buf, format, tp);
          |                                                   ^~~~~~

lib/selinux.c

@@ -109,7 +109,7 @@ int reset_selinux_file_context (void)
 /*
  *  Log callback for libselinux internal error reporting.
  */
-__attribute__((__format__ (printf, 2, 3)))
+format_attr(printf, 2, 3)
 static int selinux_log_cb (int type, const char *fmt, ...) {
 	va_list ap;
 	char *buf;