[libc] Remove unused variable causing build errors

Author: jhuber6

libc/src/__support/CPP/mutex.h

@@ -31,7 +31,7 @@ template <typename MutexType> class lock_guard {
   // Acquires ownership of the mutex object `m` without attempting to lock
   // it. The behavior is undefined if the current thread does not hold the
   // lock on `m`. Does not call `m.lock()` upon resource acquisition.
-  lock_guard(MutexType &m, adopt_lock_t t) : mutex(m) {}
+  lock_guard(MutexType &m, adopt_lock_t /* t */) : mutex(m) {}
 
   ~lock_guard() { mutex.unlock(); }