Onigumo 6 (r57045) introduced new onigumo.h header file, which is
required from quite much everywhere.  This commit adds necessary
dependencies.

Note: ruby/oniguruma.h now includes onigumo.h,
      ruby/io.h includes oniguruma.h,
      ruby/encoding.h also includes oniguruma.h,
      and internal.h includes encoding.h.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58054 b2dd03c8-39d4-4d8f-98ff-823fe69b080e