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/timer.h

@@ -4,8 +4,8 @@
  *
  * timer.h - Generic timer
  */
-#ifndef __LIBCAMERA_BASE_TIMER_H__
-#define __LIBCAMERA_BASE_TIMER_H__
+
+#pragma once
 
 #include <chrono>
 #include <stdint.h>
@@ -47,5 +47,3 @@ private:
 };
 
 } /* namespace libcamera */
-
-#endif /* __LIBCAMERA_BASE_TIMER_H__ */