fix missing import

[lekoala]

this make @var GridField works properly for static analysis

Description

phpstan complains about invalid gridfield

Manual testing steps

use phpstan and see
Parameter #2 $gridfield of class Colymba\BulkTools\HTTPBulkToolsResponse constructor expects SilverStripe\Forms\GridField\GridField, Colymba\BulkManager\BulkAction\GridField given.

Issues

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[GuySartorelli]

Can you please target the 4.0 or 4 branch? Either one is fine.

[lekoala]

@GuySartorelli i created another PR it was a nightmare to try to change this

[GuySartorelli]

Closing in favor of #277