patch 9.1.1431: Hit-Enter Prompt when loading session files

Problem:  Hit-Enter Prompt when loading session files
Solution: use set+= for 'shortmess' to keep the existing flags
          (Miguel Barro)

closes: #17445

Signed-off-by: Miguel Barro <miguel.barro@live.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/version.c b/src/version.c
index 5630b8a..0df54f2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -710,6 +710,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1431,
+/**/
     1430,
 /**/
     1429,