patch 9.0.0091: duplicate error number

Problem:    Duplicate error number.
Solution:   Use unique error number.
diff --git a/src/version.c b/src/version.c
index 2becd39..5adb215 100644
--- a/src/version.c
+++ b/src/version.c
@@ -736,6 +736,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    91,
+/**/
     90,
 /**/
     89,