Don't build :public_headers_warnings_check on NoGPU builds.

skia.h includes GPU headers.  No real point in getting fancy here... if there are no unused parameters in all headers, there are no unused parameters in the subset of non-GPU headers.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4916

Change-Id: Iaaea405633aabcc82e40cd4b1a2286aadd5cea17
Reviewed-on: https://skia-review.googlesource.com/4916
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 4c79375..7b627f1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -722,7 +722,7 @@
   }
 }
 
-if (is_skia_standalone) {
+if (is_skia_standalone && skia_enable_gpu) {
   source_set("public_headers_warnings_check") {
     sources = [
       "tools/public_headers_warnings_check.cpp",