update_engine: disable linter complaints about the unittest.cc file names

BUG=chromium:979268
TEST=none

Change-Id: I35bb6f83d5bf25f4eb2f384c4ca41a1501858379
Reviewed-on: https://chromium-review.googlesource.com/1680857
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Keigo Oka <oka@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 14c7a92..9ff26e0 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -14,6 +14,9 @@
 # limitations under the License.
 #
 
+# Stop linter from complaining XXX_unittest.cc naming.
+# gnlint: disable=GnLintSourceFileNames
+
 import("//common-mk/generate-dbus-adaptors.gni")
 import("//common-mk/generate-dbus-proxies.gni")
 import("//common-mk/openssl_pem.gni")