This change adds third_party directory to the ignore list
for the pre-submit script.

Bug: b/171873846
Change-Id: Ic1a6fee89c76e8ca3526c95e29335eacf2e505f1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49768
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Jari Komppa <jari@siru.fi>