thread: add uv_thread_cb typedef
thread: add uv_thread_cb typedef

To keep with convention, add a typedef for the function called that
initializes the new thread.

Signed-off-by: Fedor Indutny <e39d94bae70283f13d24c278e04952d6dad9384b@indutny.com>

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