chore(functions/slack): add back missing package (#3690)
chore(functions/slack): add back missing package (#3690)

Hotfix for erroneous `pom.xml` change in #3689 

@lesv please merge **when tests pass** - thanks!
GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples