umockdev-record: Fix all_devices() type conversion

Avoid directly accessing `.data()`, use the `.steal()` API instead which
does proper type coercion.

```
umockdev-record.c:418:27: note: expected ‘gchar **’ {aka ‘char **’} but argument is of type ‘void **’
../../source/src/umockdev-record.vala:64:72: error: pointer type mismatch in conditional expression [-Werror]
```

Author: martinpitt

src/umockdev-record.vala

@@ -61,7 +61,7 @@ all_devices ()
 {
     var devs = new GenericArray<string>();
     devices_from_dir("/sys/devices", ref devs);
-    return devs.data;
+    return devs.steal();
 }
 
 // If dev is a block or character device, convert it to a sysfs path.