Reset negatable switches correctly in wxCmdLineParser::Reset().

Reset negatable switches correctly in wxCmdLineParser::Reset().

The "negated" flag of wxCmdLineOption struct was not reset by Reset() so
parsing a command line with a negatable option once influenced the result of
parsing it the next time because the old value was kept.

Do clear it now to allow calling Parse() several times without side effects.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68315 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775