patch 8.1.2311: warning for missing function prototype

Problem:    Warning for missing function prototype.
Solution:   Add the proto. (Dominique Pelle, closes #5233)
diff --git a/src/version.c b/src/version.c
index 35dc4e2..35969b5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2311,
+/**/
     2310,
 /**/
     2309,