Remove some more compat padding

Looks like I missed this last one

32blit/engine/api_private.hpp

diff --git a/32blit/engine/api_private.hpp b/32blit/engine/api_private.hpp
index 6f2c20d..f45db2a 100644
--- a/32blit/engine/api_private.hpp
+++ b/32blit/engine/api_private.hpp
@@ -149,7 +149,7 @@
   };
 
   struct APIData {
-    uint16_t pad[2];
+    COMPAT_PAD(uint16_t, pad, 2);
 
     ButtonState buttons;
     float hack_left;