patch 8.1.0875: not all errors of marks and findfile()/finddir() are tested

Problem:    Not all errors of marks and findfile()/finddir() are tested.
Solution:   Add more test coverage. (Dominique Pelle)
diff --git a/src/version.c b/src/version.c
index 24a9fb3..9b2e7c9 100644
--- a/src/version.c
+++ b/src/version.c
@@ -784,6 +784,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    875,
+/**/
     874,
 /**/
     873,