Raise minimum `bitflags` version to v1.2

[daxpedda]

This is an issue uncovered by https://github.com/Smithay/wayland-rs/pull/629. calloop relies on a call to from_bits_unchecked(): https://github.com/Smithay/calloop/blob/0ba3f1e24417fdb07960e609cb65ed9b04a19c47/src/io.rs#L55 ... which is only available since bitflags v1.2, but nix only requires v1.1.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.18 :warning:

Comparison is base (0ba3f1e) 88.98% compared to head (3536f16) 88.80%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #133 +/- ## ========================================== - Coverage 88.98% 88.80% -0.18% ========================================== Files 14 14 Lines 1671 1671 ========================================== - Hits 1487 1484 -3 - Misses 184 187 +3 ``` | Flag | Coverage Δ | | |---|---|---| | macos-latest | `88.51% <ø> (-0.20%)` | :arrow_down: | | ubuntu-latest | `88.39% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay#carryforward-flags-in-the-pull-request-comment) to find out more. [see 2 files with indirect coverage changes](https://app.codecov.io/gh/Smithay/calloop/pull/133/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Smithay)

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