cleanup: Switch to pragma once

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

src/core/constraints.h

@@ -20,8 +20,7 @@
  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  */
 
-#ifndef META_CONSTRAINTS_H
-#define META_CONSTRAINTS_H
+#pragma once
 
 #include "core/frame.h"
 #include "core/window-private.h"
@@ -35,5 +34,3 @@ void meta_window_constrain (MetaWindow          *window,
                             MetaRectangle       *intermediate,
                             int                 *rel_x,
                             int                 *rel_y);
-
-#endif /* META_CONSTRAINTS_H */