Fix build file

Change-Id: I767fe366bb3484b2f21db61c9fd964172c5b0537
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31908
Reviewed-by: Saman Sami <samans@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Presubmit-Ready: Saman Sami <samans@chromium.org>
Tested-by: Saman Sami <samans@chromium.org>

src/System/BUILD.gn

diff --git a/src/System/BUILD.gn b/src/System/BUILD.gn
index 3951cc1..d72bcab 100644
--- a/src/System/BUILD.gn
+++ b/src/System/BUILD.gn
@@ -32,7 +32,6 @@
     "Resource.hpp",
     "Socket.cpp",
     "Socket.hpp",
-    "Thread.cpp",
     "Thread.hpp",
     "Timer.cpp",
     "Timer.hpp",