patch 8.1.2264: there are two test files for :let

Problem:    There are two test files for :let.
Solution:   Merge the two files.
diff --git a/src/version.c b/src/version.c
index fe0d805..8dcefd4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2264,
+/**/
     2263,
 /**/
     2262,