file.c: remove unnecessary declaration

* file.c (GetLastError): already defined in windows.h on nowadays
  cygwin, and caused the confliction with the system provided
  definition on cygwin64.  by @kou1okada [Fixes GH-433].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43583 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-11-09

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43584 b2dd03c8-39d4-4d8f-98ff-823fe69b080e