util.c: bump stack size in ruby_qsort()

* util.c (ruby_qsort): fix potential stack overflow on a large
  machine.  based on the patch by Conrad Irwin <conrad.irwin AT
  gmail.com> at [ruby-core:51816].  [Bug #7772]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44195 b2dd03c8-39d4-4d8f-98ff-823fe69b080e