backlight: Fix copy/paste error printing an unrelated error code

udev_device_get_sysattr_value returns NULL on failure, but doesn't
provide an error code; thus, when printing an error from it, don't print
an unrelated error code from a previous call.

src/backlight/backlight.c

@@ -322,7 +322,7 @@ int main(int argc, char *argv[]) {
 
                 value = udev_device_get_sysattr_value(device, "brightness");
                 if (!value) {
-                        log_error("Failed to read system attribute: %s", strerror(-r));
+                        log_error("Failed to read system attribute");
                         return EXIT_FAILURE;
                 }