Move FPDFAPI_FlateInit() prototype and friends to .h file

pdfium.gyp

 {
   'variables': {
     'pdf_use_skia%': 0,
     'conditions': [
       ['OS=="linux"', {
         'bundle_freetype%': 0,
       }, {  # On Android there's no system FreeType. On Windows and Mac, only a
             # few methods are used from it.
         'bundle_freetype%': 1,
       }],
@@ skipping 316 matching lines @@
       'dependencies': [
         'third_party/third_party.gyp:fx_lcms2',
         'third_party/third_party.gyp:fx_libjpeg',
         'third_party/third_party.gyp:fx_libopenjpeg',
         'third_party/third_party.gyp:fx_zlib',
       ],
       'ldflags': [ '-L<(PRODUCT_DIR)',],
       '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 401 matching lines @@
         'testing/embedder_test.cpp',
         'testing/embedder_test.h',
         'testing/embedder_test_mock_delegate.h',
         'testing/embedder_test_timer_handling_delegate.h',
         'testing/fx_string_testhelpers.cpp',
         'testing/fx_string_testhelpers.h',
       ],
     },
   ],
 }