Add extern crate openssl; to crawl to build on musl

[hatoo]

Crawlerをmuslでビルドしようとしたところ失敗しましたが
emk/rust-musl-builder#64 (comment)
を参考にextern crate openssl;をつけたらよくわかりませんがビルドできたのでPRを送らせていただきます

[codecov-io]

Codecov Report

Merging #213 into master will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   65.26%   65.13%   -0.14%     
==========================================
  Files          21       21              
  Lines         760      760              
  Branches      127      127              
==========================================
- Hits          496      495       -1     
  Misses        154      154              
- Partials      110      111       +1

Impacted Files	Coverage Δ
atcoder-problems-backend/src/utils.rs	82.5% <0%> (-2.5%)	⬇️

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