patch 8.1.0888: the a: dict is not immutable as documented

Problem:    The a: dict is not immutable as documented.
Solution:   Make the a:dict immutable, add a test. (Ozaki Kiichi, Yasuhiro
            Matsumoto, closes #3929)

src/version.c

@@ -783,6 +783,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    888,
 /**/
     887,
 /**/