patch 8.1.1608: the evalfunc.c file is too big

Problem:    The evalfunc.c file is too big.
Solution:   Move sign functionality to sign.c.

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1608,
 /**/
     1607,
 /**/