fix: rbsecp256k1 LoadError

[jtgi]

what:

• plugin stopped working due to 5xx hitting /discourse-siwe/message?...
• sample error from discourse production.log

  LoadError (cannot load such file -- rbsecp256k1/rbsecp256k1)
  app/controllers/application_controller.rb:414:in `block in with_resolved_locale'
  app/controllers/application_controller.rb:414:in `with_resolved_locale'
  lib/middleware/omniauth_bypass_middleware.rb:74:in `call'
  lib/content_security_policy/middleware.rb:12:in `call'
  lib/middleware/anonymous_cache.rb:367:in `call'
  config/initializers/100-quiet_logger.rb:20:in `call'
  config/initializers/100-silence_logger.rb:29:in `call'
  lib/middleware/enforce_hostname.rb:24:in `call'
  lib/middleware/request_tracker.rb:228:in `call'

fix:

• bump rbsecp256k1 to 5.1.1
• rbsecp256k1 5.1.1 in turn requires pkg-config 1.5.0

Related issue: https://github.com/spruceid/discourse-siwe-auth/issues/21

[w4ll3]

Great work!! Thanks for your contribution.