patch 8.2.3665: cannot use a lambda for 'tagfunc'

Problem:    Cannot use a lambda for 'tagfunc'.
Solution:   Use 'tagfunc' like 'opfunc'. (Yegappan Lakshmanan, closes #9204)
diff --git a/src/structs.h b/src/structs.h
index d9d3e30..bef7062 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2878,7 +2878,8 @@
     char_u	*b_p_ofu;	// 'omnifunc'
 #endif
 #ifdef FEAT_EVAL
-    char_u	*b_p_tfu;	// 'tagfunc'
+    char_u	*b_p_tfu;	// 'tagfunc' option value
+    callback_T	b_tfu_cb;	// 'tagfunc' callback
 #endif
     int		b_p_eol;	// 'endofline'
     int		b_p_fixeol;	// 'fixendofline'