configure.in: workaround of mingw-w64

* configure.in (NET_LUID): include also ifdef.h as a workaround of
  a bug in mingw-w64 header.  [ruby-core:67103] [Bug #10640]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48988 b2dd03c8-39d4-4d8f-98ff-823fe69b080e