patch 9.0.0623: error for modifying a const is not detected at compile time

Problem:    Error for modifying a const is not detected at compile time.
Solution:   Add TTFLAG_CONST and check for it in add() and extend().
diff --git a/src/version.c b/src/version.c
index 95042e8..bcc0b24 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    623,
+/**/
     622,
 /**/
     621,