gcc 12 requires additional header to build

[kxcd]

When compiling on gcc 12 (Fedora 36) we need to include the following header for this to build.

https://github.com/dashpay/bls-signatures/blob/main/src/threshold.cpp#L6

#include <memory>

Verified this issue and fix on v1.2.0.

[kwvg]

Thank you for your report! This has been resolved with #48, requesting for closure!