patch 8.1.2268: spell file flag zero is not recognized

Problem:    Spell file flag zero is not recognized.
Solution:   Use -1 as an error value, so that zero can be used as a valid flag
            number.
diff --git a/src/version.c b/src/version.c
index 3cc9879..2db75aa 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2268,
+/**/
     2267,
 /**/
     2266,