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/option.h b/src/option.h
index 9050bab..37f6dca 100644
--- a/src/option.h
+++ b/src/option.h
@@ -1068,6 +1068,9 @@
 #endif
     , BV_SW
     , BV_SWF
+#ifdef FEAT_EVAL
+    , BV_TFU
+#endif
     , BV_TAGS
     , BV_TC
     , BV_TS