fix: missing #include <mutex> in std::barrier

reference/barrier/barrier/arrive.md

@@ -47,6 +47,7 @@ arrival_token arrive(ptrdiff_t update = 1); // (1) C++26
 #include <barrier>
 #include <chrono>
 #include <iostream>
+#include <mutex>
 #include <thread>
 #include <utility>
 

reference/barrier/barrier/arrive_and_drop.md

@@ -43,6 +43,7 @@ void arrive_and_drop();
 #include <barrier>
 #include <chrono>
 #include <iostream>
+#include <mutex>
 #include <thread>
 #include <utility>
 

reference/barrier/barrier/arrive_and_wait.md

@@ -34,6 +34,7 @@ void arrive_and_wait();
 #include <barrier>
 #include <chrono>
 #include <iostream>
+#include <mutex>
 #include <thread>
 #include <utility>
 

reference/barrier/barrier/wait.md

@@ -38,6 +38,7 @@ void wait(arrival_token&& arrival) const;
 #include <barrier>
 #include <chrono>
 #include <iostream>
+#include <mutex>
 #include <thread>
 #include <utility>