libcamera: base: Convert to pragma once

Remove the verbose #ifndef/#define/#endif pattern for maintaining
header idempotency, and replace it with a simple #pragma once.

This simplifies the headers, and prevents redundant changes when
header files get moved.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>

include/libcamera/base/span.h

@@ -5,8 +5,7 @@
  * span.h - C++20 std::span<> implementation for C++11
  */
 
-#ifndef __LIBCAMERA_BASE_SPAN_H__
-#define __LIBCAMERA_BASE_SPAN_H__
+#pragma once
 
 #include <array>
 #include <iterator>
@@ -420,5 +419,3 @@ private:
 };
 
 } /* namespace libcamera */
-
-#endif /* __LIBCAMERA_BASE_SPAN_H__ */