patch 8.2.1388: Vim9: += only works for numbers

Problem:    Vim9: += only works for numbers.
Solution:   Use += as concatenate for a list. (closes #6646)
diff --git a/src/version.c b/src/version.c
index 11b6862..c95d608 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1388,
+/**/
     1387,
 /**/
     1386,