Fix imports in recurrent test

[Tobias-Fischer]

Fixes error
from src.lava.lib.dl.slayer.utils.recurrent import (
E ModuleNotFoundError: No module named 'src'

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• Issue created that explains the change and why it's needed
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Supplemental information

[bamsumit]

@Tobias-Fischer thanks for catching this fix. I am wandering how this got through our CI!