* ext/nkf/depend (nkf.o): add nkf.c as dependency.
  bsdmake tries to make nkf.o with nkf-utf8/nkf.c without this.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48510 b2dd03c8-39d4-4d8f-98ff-823fe69b080e