patch 8.1.2309: compiler warning for argument type

Problem:    Compiler warning for argument type.
Solution:   Use linenr_T and cast to varnumber_T. (John Marriott)
diff --git a/src/version.c b/src/version.c
index 00aa9e8..5030d7b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2309,
+/**/
     2308,
 /**/
     2307,