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/option.c b/src/option.c
index fe5ff43..385126e 100644
--- a/src/option.c
+++ b/src/option.c
@@ -5433,6 +5433,7 @@
 #ifdef FEAT_COMPL_FUNC
 	case PV_CFU:	return (char_u *)&(curbuf->b_p_cfu);
 	case PV_OFU:	return (char_u *)&(curbuf->b_p_ofu);
+	case PV_THSFU:	return (char_u *)&(curbuf->b_p_thsfu);
 #endif
 #ifdef FEAT_EVAL
 	case PV_TFU:	return (char_u *)&(curbuf->b_p_tfu);
@@ -5935,6 +5936,8 @@
 	    COPY_OPT_SCTX(buf, BV_CFU);
 	    buf->b_p_ofu = vim_strsave(p_ofu);
 	    COPY_OPT_SCTX(buf, BV_OFU);
+	    buf->b_p_thsfu = vim_strsave(p_thsfu);
+	    COPY_OPT_SCTX(buf, BV_THSFU);
 #endif
 #ifdef FEAT_EVAL
 	    buf->b_p_tfu = vim_strsave(p_tfu);