gn: fix fiddle

  - __SK_FORCE_IMAGE_DECODER_LINKING makes sure the PNG encoder is linked
  - updated path to make working with sample draw.cpp easier.

No public API changes.
TBR=reed@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2202353002

Review-Url: https://codereview.chromium.org/2202353002

1 files changed, 2 insertions, 0 deletions

diff --git a/BUILD.gn b/BUILD.gn
index e6defbf604..d1daccb598 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -308,7 +308,9 @@ executable("fiddle") {
   deps = [
     ":skia",
     ":skia.h",
+    ":tool_utils",
   ]
+  testonly = true
 }
 
 template("test_lib") {