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/optionstr.c b/src/optionstr.c
index ff13218..06a633b 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -271,6 +271,7 @@
 #ifdef FEAT_COMPL_FUNC
     check_string_option(&buf->b_p_cfu);
     check_string_option(&buf->b_p_ofu);
+    check_string_option(&buf->b_p_thsfu);
 #endif
 #ifdef FEAT_EVAL
     check_string_option(&buf->b_p_tfu);