パスを指定してあげることで通るようになる模様。
$ bundle config --local build.mysql2 "--with-ldflags=-L/usr/local/opt/openssl/lib --with-cppflags=-I/usr/local/opt/openssl/include"
参考
Gem native extension build fails after upgrade to MacOS 10.4 Mojave
https://github.com/brianmario/mysql2/issues/1005