curl_threads: fix MSVC compiler warning

Use LongToHandle to convert from long to HANDLE in the Win32
implementation.
This should fix the following warning when compiling with
MSVC 11 (2012) in 64-bit mode:
lib\curl_threads.c(113): warning C4306:
'type cast' : conversion from 'long' to 'HANDLE' of greater size

Closes https://github.com/curl/curl/pull/1717