Add missing header files to GN files

third_party/libpng/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/chromecast_build.gni")
 import("//build/config/arm.gni")
 
 config("libpng_config") {
   include_dirs = [ "." ]
 
@@ skipping 21 matching lines @@
     ]
   }
 }
 
 # Cannot be a static_library in component builds
 source_set("libpng_sources") {
   sources = [
     "png.c",
     "png.h",
     "pngconf.h",
+    "pngdebug.h",
     "pngerror.c",
     "pngget.c",
     "pnginfo.h",
     "pnglibconf.h",
     "pngmem.c",
     "pngpread.c",
     "pngprefix.h",
     "pngpriv.h",
     "pngread.c",
     "pngrio.c",
@@ skipping 55 matching lines @@
       ":libpng_sources",
     ]
   }
 } else {
   group("libpng") {
     public_deps = [
       ":libpng_sources",
     ]
   }
 }