ruby.h: suppress warnings

* include/ruby/ruby.h (rb_obj_wb_unprotect, rb_obj_written),
  (rb_obj_write): suppress unused-parameter warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43370 b2dd03c8-39d4-4d8f-98ff-823fe69b080e