Add rubocop as build dependency

debian/control

@@ -12,7 +12,8 @@ Build-Depends: debhelper-compat (= 13),
                redis-tools,
                ruby,
                ruby-mocha,
-               ruby-redis-client
+               ruby-redis-client,
+               rubocop
 Standards-Version: 4.7.0
 Vcs-Browser: https://salsa.debian.org/ruby-team/ruby-redis
 Vcs-Git: https://salsa.debian.org/ruby-team/ruby-redis.git