patch 8.1.1556: command for failing screenshot does not include directory

Problem:    The command displayed to show a failing screenshot does not include
            the "testdir" directory.
Solution:   Prefix the directory name so that it can be copy-pasted.
diff --git a/src/version.c b/src/version.c
index bf1a1f2..4eb37a7 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1556,
+/**/
     1555,
 /**/
     1554,