file.c: unnecessary #undef

* file.c (flock): remove unnecessary #undef flock, trace of green
  thread polling.  patched by Rohan Garg.  [Bug #10551]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48629 b2dd03c8-39d4-4d8f-98ff-823fe69b080e