[gdb] Fix flake8 warning in gdb.python/py-inf-exited-at-exit.py

With current trunk we have:
...
$ pre-commit run flake8 --all-files
flake8..................................................................Failed
- hook id: flake8
- exit code: 1

gdb.python/py-inf-exited-at-exit.py:22:1: F821 undefined name 'gdb'
...

Fix this by adding the missing import.