fix(udev_interface.h): remove return from voids

BUG=None
TEST=None

gcc is complaining about these returns, just remove them:

update_engine-0.0.1-r366: ../update_engine/udev_interface.h:190:27:
error: return-statement with a value, in function returning 'void'
[-fpermissive]

Change-Id: Iad34fa669e8cc5597f09058fdaa7c3bb3e3ec87b
Reviewed-on: https://gerrit.chromium.org/gerrit/44894
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Brandon Philips <brandon@ifup.co>
Tested-by: Brandon Philips <brandon@ifup.co>
diff --git a/udev_interface.h b/udev_interface.h
index 4b8fc17..c241567 100644
--- a/udev_interface.h
+++ b/udev_interface.h
@@ -158,7 +158,7 @@
     return udev_device_get_syspath(udev_device);
   }
   virtual void DeviceUnref(struct udev_device* udev_device) {
-    return udev_device_unref(udev_device);
+    udev_device_unref(udev_device);
   }
 
   virtual struct udev_enumerate* EnumerateNew(struct udev* udev) {
@@ -180,14 +180,14 @@
     return udev_enumerate_get_list_entry(udev_enum);
   }
   virtual void EnumerateUnref(struct udev_enumerate* udev_enum) {
-    return udev_enumerate_unref(udev_enum);
+    udev_enumerate_unref(udev_enum);
   }
 
   virtual struct udev* New() {
     return udev_new();
   }
   virtual void Unref(struct udev* udev) {
-    return udev_unref(udev);
+    udev_unref(udev);
   }
 };