configure: Set default value for plugin_write_stackdriver.

Otherwise the output is going to be something like this when the plugin
cannot be built, which is ugly:

```
    write_sensu . . . . . yes
    write_stackdriver . .
    write_tsdb  . . . . . yes
```

diff --git a/configure.ac b/configure.ac
index 5333c42..641a71a 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -6406,6 +6406,7 @@ plugin_vmem="no"
 plugin_vserver="no"
 plugin_wireless="no"
 plugin_write_prometheus="no"
+plugin_write_stackdriver="no"
 plugin_xencpu="no"
 plugin_zfs_arc="no"
 plugin_zone="no"