Fix incorrect initialization of unsafe_locker

[rkonklewski-am2m]

@Naios

This fixes #51

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What was a problem?

List-initialization of unsafe_locker class resulted in a compiler error, because 1) it had no constructor matching the arguments, and 2) it had other user-declared constructors, so aggregate initialization was not allowed (per C++20 rules).

References

• https://en.cppreference.com/w/cpp/language/list_initialization
• https://en.cppreference.com/w/cpp/language/aggregate_initialization

How this PR fixes the problem?

This change fixes the issue by adding an appropriate constructor.

Check lists (check x in [ ] of list items)

• [ ] Additional Unit Tests were added that test the feature or regression
• [x] Coding style (Clang format was applied)

Additional Comments (if any)

IMO no new unit tests were needed.