Hide more symbols

Exposed by:

$ make -j5 CPPFLAGS="-O0" CXXFLAGS=-flto=thin LDFLAGS=-lc++ && ./check-symbols.sh

src/hb-blob.hh

@@ -55,7 +55,7 @@ struct hb_blob_t
   HB_INTERNAL bool try_make_writable_inplace_unix ();
 
   template <typename Type>
-  const Type* as () const
+  HB_INTERNAL const Type* as () const
   {
     return length < hb_null_size (Type) ? &Null(Type) : reinterpret_cast<const Type *> (data);
   }