Build Expat on all platforms

* bump version to @android-6.0.1_r55 (2.1.1)

* add as a dependency to the 'xml' target (for upcoming http://crrev.com/2142893006)

* tweak build for Mac, Win:

  - define HAVE_EXPAT_CONFIG_H (same as in Android.mk)
  - use -Wno-missing-field-initializers (same as in Android.mk)
  - suppress MSVS warning 4244 (same as ..cmake/Utilities/cmexpat/expatConfig.h.in)

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2150603004
R=mtklein@google.com

Review-Url: https://codereview.chromium.org/2150603004
diff --git a/gyp/xml.gyp b/gyp/xml.gyp
index 36acf97..8060076 100644
--- a/gyp/xml.gyp
+++ b/gyp/xml.gyp
@@ -11,6 +11,7 @@
       'standalone_static_library': 1,
       'dependencies': [
         'skia_lib.gyp:skia_lib',
+        'expat.gyp:expat',
       ],
       'include_dirs': [
         '../include/private',