We do not need to maintain empty functions for GC,
gc_mark_children will not attempt to dereference NULL
dmark callbacks.

* ext/socket/ifaddr.c (ifaddr_mark): remove empty function
  (ifaddr_type): pass zero to rb_data_type_t.function.dmark

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53016 b2dd03c8-39d4-4d8f-98ff-823fe69b080e