GN: define SK_XML to indicate :xml is enabled.

GN: define SK_XML to indicate :xml is enabled.

Without this, nothing ever defines SK_XML for dm/DM.cpp, dm/DMSrcSink.cpp, tests/SkDOMTest.cpp or tests/SkPEGTest.cpp.

GYP does something similiar in gyp/xml.gyp.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4500

Change-Id: I93322722d6d86f5802495d7a2edad2f82ba36f69
Reviewed-on: https://skia-review.googlesource.com/4500
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>