Add vulkan-headers and vulkan-validation-layers to .gitignore

BUG=dawn:150

Change-Id: I6f90ebc2e4f26d07856fb6b22cd076d8d71629a4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13240
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/.gitignore b/.gitignore
index 9c42745..c27c509 100644
--- a/.gitignore
+++ b/.gitignore
@@ -21,6 +21,8 @@
 third_party/shaderc/
 third_party/spirv-cross/
 third_party/spirv-headers/
+third_party/vulkan-headers/
+third_party/vulkan-validation-layers/
 tools
 out