Fix class/struct forward declaration inconsistencies (good on ya, MSVC)

src/script/cpp_api/s_entity.h

@@ -23,8 +23,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "cpp_api/s_base.h"
 #include "irr_v3d.h"
 
-class ObjectProperties;
-class ToolCapabilities;
+struct ObjectProperties;
+struct ToolCapabilities;
 
 class ScriptApiEntity
 		: virtual public ScriptApiBase