Make XML compilation conditional.

Only enabled for gyp-based builds at this time.

R=dogben@google.com,mtklein@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2150353002

Review-Url: https://codereview.chromium.org/2150353002
diff --git a/gyp/xml.gyp b/gyp/xml.gyp
index f7cb429..6529d0c 100644
--- a/gyp/xml.gyp
+++ b/gyp/xml.gyp
@@ -35,6 +35,9 @@
         'include_dirs': [
           '../include/xml',
         ],
+        'defines' : [
+          'SK_XML',
+        ],
       },
     },
   ],