decoder: consider reset() a mandatory decoder method

In theory support for PD reset is optional, applications are not
required to make use of it. But it's essential to receive correct
decoding results when used with the popular Pulseview mainline
application.

So let's consider the absence of the reset() method fatal. All mainline
decoders have it. Out-of-tree decoders are easy to adjust, and very
probably should support reset() as well. Thus this change is considered
beneficial, and not harmful.