.gitignore: Add thirdparty/benchmark

External dependency added when you `gclient sync`.

Change-Id: I65303a35f8ac33e3864267b742f619eeacb5a454
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/77941
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>
diff --git a/.gitignore b/.gitignore
index 52db664..0503afb2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,6 +17,7 @@
 /third_party/clang-format
 /third_party/catapult
 /third_party/cpplint
+/third_party/benchmark
 /third_party/binutils
 /third_party/glslang
 /third_party/googletest