getopt.h (optreset): Define as __mingw_optreset regardless of
_BSD_SOURCE being defined. (bug #3554856.)

git-svn-id: svn+ssh://svn.code.sf.net/p/mingw-w64/code/stable/v2.x@5340 4407c894-4637-0410-b4f5-ada5f102cad1