BUILD: Use correct type for num locks

[Krzmbrzl]

The CRYPTO_num_locks function of OpenSSL returns an int, but we assigned the value to an unsigned int, leading to a compiler warning about a potential sign change. This is no issue on newer systems as apparently OpenSSL has removed these functions at some point and replaced them with a macro that evaluates to a numeric constant (1). However, on older systems such as CentOS 7, this function still seems to exist and there it actually matters that we are using a wrong type.

Fixes #6467

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[Krzmbrzl]

💚 All backports created successfully

Status	Branch	Result
✅	1.5.x

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