Move SkPngEncoder into public API

Bug: 713862
Change-Id: I45068ed39affe41ffe0f29bf42c5ea1d9b0247ba
Reviewed-on: https://skia-review.googlesource.com/15897
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 443b4e3..e35fc96 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -81,6 +81,7 @@
   "include/config",
   "include/core",
   "include/effects",
+  "include/encode",
   "include/gpu",
   "include/gpu/gl",
   "include/pathops",