Add third_party zstd linux arm64 and x64 directories to .gitignore.

Bug: 1126177
Change-Id: Iaa9faee2d54a07818397576c948d2b080bc2cc9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566830Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Stephen Roe <steveroe@google.com>
Cr-Commit-Position: refs/heads/master@{#832418}

third_party/.gitignore

@@ -265,3 +265,5 @@
 /xstream/lib/
 /xulrunner-sdk
 /zstd
+/zstd-linux-arm64
+/zstd-linux-x64