Ruby

cont.c: cleanup needless cast

It seems like leftover development step from r59557
("refactoring Fiber status").

I will make fiber_status use BITFIELD macro in a future
commit.

• cont.c (struct rb_fiber_struct): drop const from fiber_status
  (fiber_status_set): remove cast
  [ruby-core:86788] [Misc #14720]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63315 b2dd03c8-39d4-4d8f-98ff-823fe69b080e