Add /third_party/gpuweb to .gitignore

Change-Id: I0a854b633ae04029d54f4607fbb81522d0461113
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/67260
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/.gitignore b/.gitignore
index d1da2c1..c79ec54 100644
--- a/.gitignore
+++ b/.gitignore
@@ -13,6 +13,7 @@
 /third_party/glfw
 /third_party/glm
 /third_party/googletest
+/third_party/gpuweb
 /third_party/jinja2
 /third_party/jsoncpp
 /third_party/llvm-build