Add third_party/weston to .gitignore

Fixes regression in git status after CL [1]:
$ git status
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        third_party/weston/src/

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1962128

BUG=1029241
TBR=sky
NOTRY=true

Change-Id: I0ae0c757124edbc267a040935f85d203422fc57e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2054273Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741139}

third_party/.gitignore

@@ -230,6 +230,7 @@
 /webgpu-cts/src
 /webpagereplay/
 /webrtc
+/weston/src
 /widevine/cdm/chromeos
 /widevine/cdm/linux
 /widevine/cdm/mac