encoding.h: fix conflicts

* include/ruby/encoding.h (rb_{ascii8bit,utf8,usascii}_encindex): get
  rid of conflict with macros defined in internal.h.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42734 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-08-31

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42735 b2dd03c8-39d4-4d8f-98ff-823fe69b080e