win: fix -Wunused-function warnings in thread.c
win: fix -Wunused-function warnings in thread.c

PR-URL: https://github.com/libuv/libuv/pull/1882
Reviewed-By: Bartosz Sosnowski <aa4e0bd35c562885f21ad74b171c23cfc4370e31@janeasystems.com>
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso