patch 8.1.0668: no test for overstrike mode in the command line

Problem:    No test for overstrike mode in the command line.
Solution:   Add a test. (Dominique Pelle, closes #3742)
diff --git a/src/version.c b/src/version.c
index 21a4956..486db73 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    668,
+/**/
     667,
 /**/
     666,