exclude SkJumper_stages_8bit.cpp from Google3 build

Like _lowp.cpp, it's only meant to be compiled offline.

Change-Id: I0d4f7c1fd8fa880ffd084c1e332f6a33def6e26f
Reviewed-on: https://skia-review.googlesource.com/30262
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/public.bzl b/public.bzl
index 824da8c..7eabf09 100644
--- a/public.bzl
+++ b/public.bzl
@@ -119,6 +119,7 @@
 
         # Only pre-compiled into SkJumper_generated.S.
         "src/jumper/SkJumper_stages_lowp.cpp",
+        "src/jumper/SkJumper_stages_8bit.cpp",
     ],
 )