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().

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    623,
 /**/
     622,
 /**/