Fix compilation error because of missing vendor directory

[markspanbroek]

When nimble is used to add blscurve to a nim project, the following error appears:

gcc: error: ../../vendor/blst/build/assembly.S: No such file or directory
gcc: fatal error: no input files
compilation terminated.
gcc: error: ../../vendor/blst/src/server.c: No such file or directory
gcc: fatal error: no input files
compilation terminated.

This PR adds the 'vendor' directory to the list of directories that should be installed, fixing the issue.

[kdeme]

Cool, I encountered that issue also when working locally (outside of nimbus-eth2 repository) using nimble and in CI at https://github.com/status-im/nim-ssz-serialization/pull/1

[mratsim]

@markspanbroek you need to setup your GPG keys on Github to contribute to the repo (or commit directly from the Web UI)

• https://docs.github.com/en/github/authenticating-to-github/managing-commit-signature-verification/adding-a-new-gpg-key-to-your-github-account
• https://docs.github.com/en/github/authenticating-to-github/managing-commit-signature-verification/telling-git-about-your-signing-key

[markspanbroek]

Thanks @mratsim, I updated the commit to include a GPG signature.