patch 8.2.3520: cannot define a function for thesaurus completion

Problem:    Cannot define a function for thesaurus completion.
Solution:   Add 'thesaurusfunc'. (Yegappan Lakshmanan, closes #8987,
            closes 8950)
diff --git a/src/structs.h b/src/structs.h
index 83a13a7..021206e 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2864,6 +2864,7 @@
 #ifdef FEAT_COMPL_FUNC
     char_u	*b_p_cfu;	// 'completefunc'
     char_u	*b_p_ofu;	// 'omnifunc'
+    char_u	*b_p_thsfu;	// 'thesaurusfunc'
 #endif
 #ifdef FEAT_EVAL
     char_u	*b_p_tfu;	// 'tagfunc'