Explicitly set version for qasync in requirements.txt

[gliu20]

Newer versions of qasync greater than 0.25.0 break with the following error

File "llama-runner/main.py", line 107, in main
    asyncio.set_event_loop_policy(qasync.DefaultQEventLoopPolicy())
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'qasync' has no attribute 'DefaultQEventLoopPolicy'

Pinning to an older version resolves the issue.