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.

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1356,
 /**/
     1355,
 /**/