constify rb_encoding and OnigEncoding

* include/ruby/encoding.h: constify `rb_encoding` arguments.
* include/ruby/oniguruma.h: constify `OnigEncoding` arguments.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46309 b2dd03c8-39d4-4d8f-98ff-823fe69b080e