output.compact.output(): fix ref to output.highstate.output()

Seems to have been accidentally broken when adjusting the style of the module's imports.

************* Module salt.output.compact
salt/output/compact.py:34: [E1101(no-member), output] Module 'salt.output.highstate' has no '_output' member
Merge pull request #14750 from hulu/fix-compact-output

output.compact.output(): fix ref to output.highstate.output()