Disable unused function warning

In file included from ../../third_party/swiftshader/third_party/llvm-subzero/lib/Support/Signals.cpp:183:
    ../../third_party/swiftshader/third_party/llvm-subzero/lib/Support/Windows/Signals.inc(220,13): error: unused function 'printStackTraceWithLLVMSymbolizer' [-Werror,-Wunused-function]
    static bool printStackTraceWithLLVMSymbolizer(llvm::raw_ostream &OS,
    ^
    1 error generated.

Bug: chromium build fix
Change-Id: I9ed9ac680fb5286fda0685788c6dbf76a4e34369
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39908
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/src/Reactor/BUILD.gn b/src/Reactor/BUILD.gn
index 1d958e7..9cb4b1e 100644
--- a/src/Reactor/BUILD.gn
+++ b/src/Reactor/BUILD.gn
@@ -152,6 +152,7 @@
         "-Wno-header-hygiene",
         "-Wno-enum-compare-switch",
         "-Wno-unused-lambda-capture",
+        "-Wno-unused-function",
       ]
     }