patch 8.1.0042: if omni completion opens a window Insert mode is stopped

Problem:    If omni completion opens a window Insert mode is stopped.
            (Hirohito Higashi)
Solution:   Only set stop_insert_mode in a prompt buffer window.
diff --git a/src/version.c b/src/version.c
index 71283ed..4ab241e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    42,
+/**/
     41,
 /**/
     40,