patch 8.0.0709: libvterm cannot use vsnprintf()

Problem:    Libvterm cannot use vsnprintf(), it does not exist in C90.
Solution:   Use vim_vsnprintf() instead.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    709,
 /**/
     708,
 /**/