[atomic] Fix cast to fallback ptr_get()

src/hb-atomic-private.hh

diff --git a/src/hb-atomic-private.hh b/src/hb-atomic-private.hh
index f60c46e..852b224 100644
--- a/src/hb-atomic-private.hh
+++ b/src/hb-atomic-private.hh
@@ -229,7 +229,7 @@
 };
 
 
-#define hb_atomic_ptr_get(P) hb_atomic_ptr_impl_get(P)
+#define hb_atomic_ptr_get(P) hb_atomic_ptr_impl_get((void **) P)
 #define hb_atomic_ptr_cmpexch(P,O,N) hb_atomic_ptr_impl_cmpexch((P),(O),(N))