patch 8.1.1228: not possible to process tags with a function

Problem:    Not possible to process tags with a function.
Solution:   Add tagfunc() (Christian Brabandt, Andy Massimino, closes #4010)
diff --git a/src/buffer.c b/src/buffer.c
index 5a30aff..284addf 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2219,6 +2219,9 @@
     clear_string_option(&buf->b_p_path);
     clear_string_option(&buf->b_p_tags);
     clear_string_option(&buf->b_p_tc);
+#ifdef FEAT_EVAL
+    clear_string_option(&buf->b_p_tfu);
+#endif
 #ifdef FEAT_INS_EXPAND
     clear_string_option(&buf->b_p_dict);
     clear_string_option(&buf->b_p_tsr);