patch 9.0.1966: configure prints stray 6 when checking libruby

Problem:  configure prints stray 6 when checking libruby
Solution: redirect stdout to dev/null

configure: Do not print "6" when checking for libruby

`expr` will print the matched string length to the standard output.
Current `configure` output looks like this:

```
checking Ruby header files...  /usr/include/ruby-3.1.0
6
```

The script really only cares about `expr` exit code.

closes: #13234

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Illia Bobyr <illia.bobyr@gmail.com>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1966,
 /**/
     1965,
 /**/