LibCore: Add peer pid retrieval for LocalSocket

This will allow programs connected over unix sockets to retrieve the
pid of their peers in a nice way.

Userland/Libraries/LibCore/LocalSocket.h

@@ -16,6 +16,7 @@ public:
     virtual ~LocalSocket() override;
 
     static RefPtr<LocalSocket> take_over_accepted_socket_from_system_server(String const& socket_path = String());
+    pid_t peer_pid() const;
 
 private:
     explicit LocalSocket(Object* parent = nullptr);