cmake/win32: do not compile evthread_win32 on -DEVENT__DISABLE_THREAD_SUPPORT=ON There is duplicated "evthread_win32.c" appending to ${SRC_CORE} list, leave only one that under if EVENT__DISABLE_THREAD_SUPPORT Fixes: https://ci.appveyor.com/project/azat/libevent/build/2.1.5.96/job/44q3rgifasny7gek