patch 8.1.1356: some text in heredoc assignment ends the text

Problem:    Some text in heredoc assignment ends the text. (Ozaki Kiichi)
Solution:   Recognize "let v =<<" and skip until the end.
diff --git a/src/version.c b/src/version.c
index 042a8bc..2dd024f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -768,6 +768,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1356,
+/**/
     1355,
 /**/
     1354,