patch 8.0.1557: printf() does not work with only one argument

Problem:    printf() does not work with only one argument. (Daniel Hahler)
Solution:   Allow using just the format. (Ken Takata, closes #2687)
diff --git a/src/version.c b/src/version.c
index e1f406a..7d54185 100644
--- a/src/version.c
+++ b/src/version.c
@@ -779,6 +779,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1557,
+/**/
     1556,
 /**/
     1555,