Add missing option to the default configuration file.

While the userDescription field is implemented in the code, the same
option is not present in the sample configuration file. This patch
fixes the issue by adding the option with an empty default value.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reported-by: Rohrer Hansjoerg <hj.rohrer@mobatime.com>

default.cfg

@@ -63,3 +63,4 @@ time_stamping		hardware
 productDescription	;;
 revisionData		;;
 manufacturerIdentity	00:00:00
+userDescription		;