patch 9.0.1591: some "gomod" files are not recognized

Problem:    Some "gomod" files are not recognized.
Solution:   Check for "go.mod" file name before checking out the contents.
            (Omar El Halabi, closes #12462)
diff --git a/src/version.c b/src/version.c
index fbe684a..ad1b686 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1591,
+/**/
     1590,
 /**/
     1589,