Add kjellander as BUILD.gn OWNER in webrtc/modules

NOTRY=True

Review-Url: https://codereview.webrtc.org/2258593003
Cr-Commit-Position: refs/heads/master@{#13812}

webrtc/modules/OWNERS

@@ -4,3 +4,5 @@ per-file *.isolate=kjellander@webrtc.org
 # structural changes, please get a review from a reviewer in this file.
 per-file *.gyp=*
 per-file *.gypi=*
+
+per-file BUILD.gn=kjellander@webrtc.org