Replaced `typeof` with `__typeof__` in `OnePasswordExtension.m`

This should resolve #124.

**Note to self:** Once this PR is merged we need to "cherrypick" the changes to the `add-low-level-api` branch
Merge pull request #125 from AgileBits/task/fix-typeofs

Replaced `typeof` with `__typeof__` in `OnePasswordExtension.m`