GN: re-run echo_headers if git state changes.

This should mean it re-runs if a header is added or removed.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2206683002

Review-Url: https://codereview.chromium.org/2206683002

BUILD.gn

@@ -289,6 +289,7 @@ action("skia.h") {
   script = "gn/echo_headers.py"
   args = [ rebase_path("$target_gen_dir/skia.h", root_build_dir) ] +
          rebase_path(skia_public_includes, root_build_dir)
+  inputs = [ ".git/logs/HEAD" ]   # ~~> Run any time GIT state changes.
   outputs = [
     "$target_gen_dir/skia.h",
   ]