patch 8.2.1388: Vim9: += only works for numbers

Problem:    Vim9: += only works for numbers.
Solution:   Use += as concatenate for a list. (closes #6646)

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1388,
 /**/
     1387,
 /**/