[pool] Fix alignment assertion

I *think* it should fix https://github.com/harfbuzz/harfbuzz/issues/1901

Ie. if on a system, alignof(void*) < sizeof(void*)...

src/hb-pool.hh

diff --git a/src/hb-pool.hh b/src/hb-pool.hh
index ff0ee19..2dd8496 100644
--- a/src/hb-pool.hh
+++ b/src/hb-pool.hh
@@ -77,7 +77,7 @@
 
   static_assert (ChunkLen > 1, "");
   static_assert (sizeof (T) >= sizeof (void *), "");
-  static_assert (alignof (T) % sizeof (void *) == 0, "");
+  static_assert (alignof (T) % alignof (void *) == 0, "");
 
   struct chunk_t
   {