Update Changelog

[jonasnick]

Looking at the PRs since the last release, the following ones seem to deserve a changelog entry:

• Drop src/libsecp256k1-config.h #1178
• build: Add SECP256K1_API_VAR to fix importing variables from DLLs #1209
• maybe Prevent dead-store elimination when clearing secrets in examples #1212

[real-or-random]

Thanks, I was also about to make a list. What are your thoughts on adding #1188? This can be interesting for users who run the tests manually instead of using make check. I guess there are a few reasons to do this.

• maybe Prevent dead-store elimination when clearing secrets in examples #1212

I don't think it's strictly necessary, but I'm happy to include it. It won't hurt to draw the attention of users to this.