Ignore third party libraries during presubmit

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>
diff --git a/tests/check_build_files/main.go b/tests/check_build_files/main.go
index 8ab972d..7f2f5a5 100644
--- a/tests/check_build_files/main.go
+++ b/tests/check_build_files/main.go
@@ -63,7 +63,7 @@
 		}
 
 		switch rel {
-		case ".git", "cache", "build", "out":
+		case ".git", "cache", "build", "out", "third_party":
 			return filepath.SkipDir
 		}