Add missing imports in varsign.go

[robertfairhead]

A recent commit added package references to varsign.go but did not update the file with those import statements.

This caused our CI to fail when we imported the updated version of the package.

This adds the missing import statements.

[Foxboron]

Ah, that is because I'm doing git add --patch in an attempt to get tidy commits and I'm not running anything over the commited files to check they are valid :upside_down_face:

But considering people are pulling down my stuff I should probably fix that. Thanks!