Enabling 'gn check' on webrtc/sdk

BUG=webrtc:7499

Review-Url: https://codereview.webrtc.org/2818433003
Cr-Commit-Position: refs/heads/master@{#17805}
diff --git a/.gn b/.gn
index edfad1a..1265bc5 100644
--- a/.gn
+++ b/.gn
@@ -33,6 +33,7 @@
   "//webrtc/media/*",
   "//webrtc/modules/*",
   "//webrtc/ortc/*",
+  "//webrtc/sdk/*",
   "//webrtc/stats/*",
   "//webrtc/tools/*",
   "//webrtc/test/*",