dev: fix interface implement check for "SkipDirs"

[shmsr]

For every processor, there's a compile-time check for processors to check if they implement the Processor interface:

type Processor interface {
	Process(issues []result.Issue) ([]result.Issue, error)
	Name() string
	Finish()
}

But there's a typo: SkipFiles is getting checked instead of SkipDirs

Fix:

var _ Processor = (*SkipDirs)(nil)

(or)

var _ Processor = &SkipDirs

The former is more idiomatic; so I did that.

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[shmsr]

@ldez Good for merge?

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