patch 8.0.0709: libvterm cannot use vsnprintf()

Problem:    Libvterm cannot use vsnprintf(), it does not exist in C90.
Solution:   Use vim_vsnprintf() instead.
diff --git a/src/version.c b/src/version.c
index b4e7d2e..e284648 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    709,
+/**/
     708,
 /**/
     707,