Add libffi-devel before bundle

Related issue: #5 (closed)

Add libffi-devel before bundle install.

Failed job case: https://jihulab.com/gitlab-cn/code-sync/-/jobs/5892395

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
current directory:
/builds/gitlab-cn/code-sync/gitlab/vendor/ruby/2.7.0/gems/llhttp-ffi-0.4.0/ext
/usr/local/bin/ruby -I/usr/local/lib/ruby/site_ruby/2.7.0 -rrubygems
/builds/gitlab-cn/code-sync/gitlab/vendor/ruby/2.7.0/gems/rake-13.0.6/exe/rake
RUBYARCHDIR\=/builds/gitlab-cn/code-sync/gitlab/vendor/ruby/2.7.0/extensions/x86_64-linux/2.7.0/llhttp-ffi-0.4.0
RUBYLIBDIR\=/builds/gitlab-cn/code-sync/gitlab/vendor/ruby/2.7.0/extensions/x86_64-linux/2.7.0/llhttp-ffi-0.4.0
rake aborted!
LoadError: libffi.so.6: cannot open shared object file: No such file or
directory -
/builds/gitlab-cn/code-sync/gitlab/vendor/ruby/2.7.0/gems/ffi-1.15.5/lib/ffi_c.so

An error occurred while installing llhttp-ffi (0.4.0), and Bundler cannot
continue.
In Gemfile:
  kubeclient was resolved to 4.9.4.pre.gitlab1, which depends on
    http was resolved to 5.1.1, which depends on
      llhttp-ffi