[botan] Fix build error C2039 with Visual Studio 2019 and C++17

[LilyWangL]

Fix build error:

/botan/src/5a35da0742-04e84f84a0/src/lib/utils/filesystem.cpp(41): error C2039: 'tr2': is not a member of 'std'
/botan/src/5a35da0742-04e84f84a0/src/lib/utils/filesystem.cpp(41): error C3083: 'tr2': the symbol to the left of a '::' must be a type
/botan/src/5a35da0742-04e84f84a0/src/lib/utils/filesystem.cpp(41): error C2039: 'sys': is not a member of 'std'
/botan/src/5a35da0742-04e84f84a0/src/lib/utils/filesystem.cpp(41): error C2871: 'sys': a namespace with this name does not exist
/botan/src/5a35da0742-04e84f84a0/src/lib/utils/filesystem.cpp(46): error C2065: 'path': undeclared identifier