* io.c (linux_iocparm_len): enable only exist _IOC_SIZE().
  Because musl libc doesn't have it. [Bug #8051] [ruby-core:53229]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39818 b2dd03c8-39d4-4d8f-98ff-823fe69b080e