LibThreading: Add ConditionVariable wrapper

ConditionVariable is a thin wrapper over the pthread_cond_* APIs, just
as Mutex is a wrapper over pthread_mutex.

Because ConditionVariable might want to wait on a high-level Mutex, it
needs to be friends with it.

Userland/Libraries/LibThreading/Mutex.h

@@ -13,6 +13,8 @@
 namespace Threading {
 
 class Mutex {
+    friend class ConditionVariable;
+
 public:
     Mutex()
     {