Better hide nil objects and make them const

src/hb-icu.cc

diff --git a/src/hb-icu.cc b/src/hb-icu.cc
index 5cd0143..78cbc1a 100644
--- a/src/hb-icu.cc
+++ b/src/hb-icu.cc
@@ -269,23 +269,22 @@
   return ret;
 }
 
-extern HB_INTERNAL hb_unicode_funcs_t _hb_unicode_funcs_icu;
-hb_unicode_funcs_t _hb_icu_unicode_funcs = {
-  HB_OBJECT_HEADER_STATIC,
-
-  NULL, /* parent */
-  TRUE, /* immutable */
-  {
-#define HB_UNICODE_FUNC_IMPLEMENT(name) hb_icu_unicode_##name,
-    HB_UNICODE_FUNCS_IMPLEMENT_CALLBACKS
-#undef HB_UNICODE_FUNC_IMPLEMENT
-  }
-};
-
 hb_unicode_funcs_t *
 hb_icu_get_unicode_funcs (void)
 {
-  return &_hb_icu_unicode_funcs;
+  static const hb_unicode_funcs_t _hb_icu_unicode_funcs = {
+    HB_OBJECT_HEADER_STATIC,
+
+    NULL, /* parent */
+    TRUE, /* immutable */
+    {
+#define HB_UNICODE_FUNC_IMPLEMENT(name) hb_icu_unicode_##name,
+      HB_UNICODE_FUNCS_IMPLEMENT_CALLBACKS
+#undef HB_UNICODE_FUNC_IMPLEMENT
+    }
+  };
+
+  return const_cast<hb_unicode_funcs_t *> (&_hb_icu_unicode_funcs);
 }