fix MSVC warning with -W4

[sthalik]

In Containers::BitArray, cast bool to char before performing arithmetic on it.

[codecov[bot]]

Codecov Report

Base: 97.94% // Head: 97.94% // No change to project coverage :thumbsup:

Coverage data is based on head (32c181c) compared to base (14bd297). Patch coverage: 100.00% of modified lines in pull request are covered.

:exclamation: Current head 32c181c differs from pull request most recent head 04be172. Consider uploading reports for the commit 04be172 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #151 +/- ## ======================================= Coverage 97.94% 97.94% ======================================= Files 133 133 Lines 10862 10862 ======================================= Hits 10639 10639 Misses 223 223 ``` | [Impacted Files](https://codecov.io/gh/mosra/corrade/pull/151?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Vladim%C3%ADr+Vondru%C5%A1) | Coverage Δ | | |---|---|---| | [src/Corrade/Containers/BitArray.h](https://codecov.io/gh/mosra/corrade/pull/151/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Vladim%C3%ADr+Vondru%C5%A1#diff-c3JjL0NvcnJhZGUvQ29udGFpbmVycy9CaXRBcnJheS5o) | `92.30% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Vladim%C3%ADr+Vondru%C5%A1). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Vladim%C3%ADr+Vondru%C5%A1)

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[pezcode]

The same warning exists in BitArrayView, should probably be fixed there as well

[mosra]

Thank you both! :+1: