patch 9.0.0906: mouse scroll code is not optimal

Problem:    Mouse scroll code is not optimal.
Solution:   Properly organise Normal mode, Insert mode and common code.
            (Christopher Plewright, closes #11572)
diff --git a/src/version.c b/src/version.c
index d91a417..a87a378 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    906,
+/**/
     905,
 /**/
     904,