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
diff --git a/tools/fiddle/fiddle_main.cpp b/tools/fiddle/fiddle_main.cpp
index 1c80c44..df2e8ff 100644
--- a/tools/fiddle/fiddle_main.cpp
+++ b/tools/fiddle/fiddle_main.cpp
@@ -7,6 +7,9 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include "SkForceLinking.h"
+
+__SK_FORCE_IMAGE_DECODER_LINKING;
 
 #include "fiddle_main.h"