compose.py: Fix error when integration is false
I was getting the following when I set config.integration to False. This is the smallest change required to fix the problem, but it might benefit for being looked at to make sure it's all being sane.
[--:--:--] BUG [debian-python-split.bst]: Build An unhandled exception occured: Traceback (most recent call last): File "/home/jonathanmaw/workspace/buildstream/buildstream/buildstream/_scheduler/job.py", line 255, in child_action result = self.action(element) File "/home/jonathanmaw/workspace/buildstream/buildstream/buildstream/_scheduler/buildqueue.py", line 34, in process element._assemble() File "/home/jonathanmaw/workspace/buildstream/buildstream/buildstream/element.py", line 768, in _assemble collect = self.assemble(sandbox) File "/home/jonathanmaw/workspace/buildstream/buildstream/buildstream/plugins/elements/compose.py", line 125, in assemble manifest += integration_files UnboundLocalError: local variable 'integration_files' referenced before assignment