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.h b/src/option.h
index 48a3ec3..75c83d5 100644
--- a/src/option.h
+++ b/src/option.h
@@ -404,6 +404,7 @@
 #ifdef FEAT_COMPL_FUNC
 EXTERN char_u	*p_cfu;		// 'completefunc'
 EXTERN char_u	*p_ofu;		// 'omnifunc'
+EXTERN char_u	*p_thsfu;	// 'thesaurusfunc'
 #endif
 EXTERN int	p_ci;		// 'copyindent'
 #if defined(FEAT_GUI) && defined(MACOS_X)
@@ -1217,6 +1218,9 @@
 #endif
     , BV_TAGS
     , BV_TC
+#ifdef FEAT_COMPL_FUNC
+    , BV_THSFU
+#endif
     , BV_TS
     , BV_TW
     , BV_TX