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/flags.h

@@ -4,8 +4,8 @@
  *
  * flags.h - Type-safe enum-based bitfields
  */
-#ifndef __LIBCAMERA_BASE_FLAGS_H__
-#define __LIBCAMERA_BASE_FLAGS_H__
+
+#pragma once
 
 #include <type_traits>
 
@@ -191,5 +191,3 @@ struct flags_enable_operators<_enum> {					\
 #endif /* __DOXYGEN__ */
 
 } /* namespace libcamera */
-
-#endif /* __LIBCAMERA_BASE_FLAGS_H__ */