windows: fix conflicted declaration in MinGW enrironment
windows: fix conflicted declaration in MinGW enrironment

This change fixes following error:

  /usr/lib/gcc/i686-w64-mingw32/4.6/../../../../i686-w64-mingw32/include/stdio.h:350:5:
  error: previous declaration of 'int _vsnprintf(char*, size_t, const
  char*, char*)' with 'C++' linkage
  /usr/lib/gcc/i686-w64-mingw32/4.6/../../../../i686-w64-mingw32/include/stdio.h:534:121:
  error: conflicts with new declaration with 'C' linkage

groonga/groonga,cosmo0920/groonga,redfigure/groonga,komainu8/groonga,naoa/groonga,myokoym/groonga,komainu8/groonga,groonga/groonga,redfigure/groonga,hiroyuki-sato/groonga,naoa/groonga,kenhys/groonga,naoa/groonga,myokoym/groonga,kenhys/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,groonga/groonga,hiroyuki-sato/groonga,groonga/groonga,myokoym/groonga,myokoym/groonga,myokoym/groonga,hiroyuki-sato/groonga,redfigure/groonga,groonga/groonga,hiroyuki-sato/groonga,naoa/groonga,myokoym/groonga,myokoym/groonga,cosmo0920/groonga,redfigure/groonga,komainu8/groonga,komainu8/groonga,redfigure/groonga,komainu8/groonga,kenhys/groonga,redfigure/groonga,cosmo0920/groonga,komainu8/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,groonga/groonga,naoa/groonga,cosmo0920/groonga,naoa/groonga,hiroyuki-sato/groonga,kenhys/groonga,hiroyuki-sato/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,cosmo0920/groonga,redfigure/groonga,myokoym/groonga,komainu8/groonga,cosmo0920/groonga,groonga/groonga,redfigure/groonga,komainu8/groonga