libstdc++: Add missing return for atomic timed wait [PR102074]

This adds a missing return statement to the non-futex wait-until
operation.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/102074
	* include/bits/atomic_timed_wait.h (__timed_waiter_pool)
	[!_GLIBCXX_HAVE_PLATFORM_TIMED_WAIT]: Add missing return.

libstdc++-v3/include/bits/atomic_timed_wait.h

@@ -213,6 +213,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      lock_guard<mutex> __l(_M_mtx);
 	      return __cond_wait_until(_M_cv, _M_mtx, __atime);
 	    }
+	  else
+	    return true;
 #endif // _GLIBCXX_HAVE_PLATFORM_TIMED_WAIT
 	}
     };