undef poll at the end if CPPHTTPLIB_USE_POLL (#1427)
undef poll at the end if CPPHTTPLIB_USE_POLL (#1427)

* undef poll at the end if CPPHTTPLIB_USE_POLL

* win32 only
yhirose/cpp-httplib,yhirose/cpp-httplib,yhirose/cpp-httplib,yhirose/cpp-httplib