LibThreading: Remove unused include of <sys/cdefs.h>

This file is not needed here, and causes a compile issue on musl-libc
based distributions. We should only be including this file in LibC, for
the most part anyway.

Userland/Libraries/LibThreading/ConditionVariable.h

@@ -9,7 +9,6 @@
 #include <AK/Function.h>
 #include <LibThreading/Mutex.h>
 #include <pthread.h>
-#include <sys/cdefs.h>
 #include <sys/types.h>
 
 namespace Threading {