Add back OWNERS entris that went missing

The OWNERS entries for style-guide.md were removed in
https://webrtc-review.googlesource.com/1560, apparently by accident;
add them back.

And while we're at it, add an entry for native-api.md as well.

Bug: none
Change-Id: I924b9e8dab297e668055af3042ac70123e4c00dc
Notry: true
Reviewed-on: https://webrtc-review.googlesource.com/60001
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22306}

OWNERS

@@ -15,3 +15,6 @@ per-file BUILD.gn=phoglund@webrtc.org
 per-file DEPS=*
 per-file pylintrc=phoglund@webrtc.org
 per-file WATCHLISTS=*
+per-file style-guide.md=danilchap@webrtc.org
+per-file style-guide.md=kwiberg@webrtc.org
+per-file native-api.md=kwiberg@webrtc.org