Move FPDFAPI_FlateInit() prototype and friends to .h file

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("//testing/test.gni")
 
 pdf_use_skia = false
 
 declare_args() {
   # On Android there's no system FreeType. On Windows and Mac, only a few
@@ skipping 325 matching lines @@
 static_library("fxcodec") {
   deps = [
     "third_party:fx_lcms2",
     "third_party:fx_libjpeg",
     "third_party:fx_libopenjpeg",
     "third_party:fx_zlib",
   ]
   sources = [
     "core/include/fxcodec/fx_codec.h",
     "core/include/fxcodec/fx_codec_def.h",
+    "core/include/fxcodec/fx_codec_flate.h",
     "core/include/fxcodec/fx_codec_provider.h",
     "core/src/fxcodec/codec/codec_int.h",
     "core/src/fxcodec/codec/fx_codec.cpp",
     "core/src/fxcodec/codec/fx_codec_fax.cpp",
     "core/src/fxcodec/codec/fx_codec_flate.cpp",
     "core/src/fxcodec/codec/fx_codec_icc.cpp",
     "core/src/fxcodec/codec/fx_codec_jbig.cpp",
     "core/src/fxcodec/codec/fx_codec_jbig_enc.cpp",
     "core/src/fxcodec/codec/fx_codec_jpeg.cpp",
     "core/src/fxcodec/codec/fx_codec_jpx_opj.cpp",
@@ skipping 419 matching lines @@
     "//v8",
     "//v8/include",
     "."
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_config",
     "//build/config/compiler:no_chromium_code",
   ]
 }