[amalgam] Fix UCD issue

This actually makes it build now!

Part https://github.com/harfbuzz/harfbuzz/issues/1809

Keeping open to add tests, CI, etc.
diff --git a/src/hb-unicode.hh b/src/hb-unicode.hh
index 021fa46..9b181c1 100644
--- a/src/hb-unicode.hh
+++ b/src/hb-unicode.hh
@@ -395,4 +395,7 @@
 _hb_unicode_is_emoji_Extended_Pictographic (hb_codepoint_t cp);
 
 
+extern "C" HB_INTERNAL hb_unicode_funcs_t *hb_ucd_get_unicode_funcs ();
+
+
 #endif /* HB_UNICODE_HH */