cleanup: Switch to pragma once

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

src/backends/meta-backend-types.h

@@ -18,8 +18,7 @@
  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef META_BACKEND_TYPE_H
-#define META_BACKEND_TYPE_H
+#pragma once
 
 #include <stdint.h>
 #include <stddef.h>
@@ -97,5 +96,3 @@ typedef struct _MetaInputCaptureSession MetaInputCaptureSession;
 
 typedef struct _MetaEis MetaEis;
 typedef struct _MetaEisClient MetaEisClient;
-
-#endif /* META_BACKEND_TYPE_H */