event/BackendEvents: relicense to BSD-2

These headers contain only trivial integer constants and these are
probably not even copyrightable.

src/event/BackendEvents.hxx

@@ -1,8 +1,6 @@
-// SPDX-License-Identifier: GPL-2.0-or-later
-// Copyright The Music Player Daemon Project
+// SPDX-License-Identifier: BSD-2-Clause
 
-#ifndef EVENT_BACKEND_EVENTS_HXX
-#define EVENT_BACKEND_EVENTS_HXX
+#pragma once
 
 #include "event/Features.h"
 
@@ -22,5 +20,3 @@ using EventPollBackendEvents = EpollEvents;
 using EventPollBackendEvents = PollEvents;
 
 #endif
-
-#endif

src/event/EpollEvents.hxx

@@ -1,8 +1,6 @@
-// SPDX-License-Identifier: GPL-2.0-or-later
-// Copyright The Music Player Daemon Project
+// SPDX-License-Identifier: BSD-2-Clause
 
-#ifndef EVENT_EPOLL_EVENTS_HXX
-#define EVENT_EPOLL_EVENTS_HXX
+#pragma once
 
 #include <sys/epoll.h>
 
@@ -12,5 +10,3 @@ struct EpollEvents {
 	static constexpr unsigned ERROR = EPOLLERR;
 	static constexpr unsigned HANGUP = EPOLLHUP;
 };
-
-#endif

src/event/PollEvents.hxx

@@ -1,8 +1,6 @@
-// SPDX-License-Identifier: GPL-2.0-or-later
-// Copyright The Music Player Daemon Project
+// SPDX-License-Identifier: BSD-2-Clause
 
-#ifndef EVENT_POLL_EVENTS_HXX
-#define EVENT_POLL_EVENTS_HXX
+#pragma once
 
 #include <sys/poll.h>
 
@@ -12,5 +10,3 @@ struct PollEvents {
 	static constexpr unsigned ERROR = POLLERR;
 	static constexpr unsigned HANGUP = POLLHUP;
 };
-
-#endif

src/event/WinSelectEvents.hxx

@@ -1,8 +1,6 @@
-// SPDX-License-Identifier: GPL-2.0-or-later
-// Copyright The Music Player Daemon Project
+// SPDX-License-Identifier: BSD-2-Clause
 
-#ifndef EVENT_WINSELECT_EVENTS_HXX
-#define EVENT_WINSELECT_EVENTS_HXX
+#pragma once
 
 #include <windows.h>
 
@@ -18,5 +16,3 @@ struct WinSelectEvents {
 	static constexpr unsigned ERROR = 0;
 	static constexpr unsigned HANGUP = 0;
 };
-
-#endif