cleanup: Switch to pragma once

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3157>

cogl/cogl/cogl-attribute-buffer.h

@@ -31,13 +31,12 @@
  *   Robert Bragg <robert@linux.intel.com>
  */
 
+#pragma once
+
 #if !defined(__COGL_H_INSIDE__) && !defined(COGL_COMPILATION)
 #error "Only <cogl/cogl.h> can be included directly."
 #endif
 
-#ifndef __COGL_ATTRIBUTE_BUFFER_H__
-#define __COGL_ATTRIBUTE_BUFFER_H__
-
 /* We forward declare the CoglAttributeBuffer type here to avoid some circular
  * dependency issues with the following headers.
  */
@@ -137,5 +136,3 @@ cogl_is_attribute_buffer (void *object);
 
 G_END_DECLS
 
-#endif /* __COGL_ATTRIBUTE_BUFFER_H__ */
-