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