Prefer using pragma once

Similar to what we did in mutter

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3507>

src/st/st-scrollable.h

@@ -22,8 +22,7 @@
 #error "Only <st/st.h> can be included directly.h"
 #endif
 
-#ifndef __ST_SCROLLABLE_H__
-#define __ST_SCROLLABLE_H__
+#pragma once
 
 #include <glib-object.h>
 #include <st/st-adjustment.h>
@@ -55,5 +54,3 @@ void st_scrollable_get_adjustments (StScrollable  *scrollable,
                                     StAdjustment **vadjustment);
 
 G_END_DECLS
-
-#endif /* __ST_SCROLLABLE_H__ */