popn: implement bi3a-hook (bio3) (#626)

## Link to GitHub Issue or related Pull Request, if one exists
#618 

## Description of change
Turns out running this with bi2x doesn't process buttons properly, so we
have to use bi3a.

## Testing
Seems playable now.

src/spice2x/games/popn/bi3a_hook.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#if SPICE64
+
+namespace games::popn {
+    void bi3a_hook_init();
+}
+
+#endif