simple fix for ozone w/ headless

Ozone BUILD.gn files for each platform have this line in them except headless.
Now it has it too. Still no head.

It was causing double definition of some symbols when building for windows.

Bug:

Change-Id: I3455ab67848f05c0b3596a53cf6f6279bf078f0c
Reviewed-on: https://chromium-review.googlesource.com/963227Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Mustafa Çamurcu <camurcu@google.com>
Cr-Commit-Position: refs/heads/master@{#543432}

ui/ozone/platform/headless/BUILD.gn

@@ -20,6 +20,8 @@ source_set("headless") {
     "ozone_platform_headless.h",
   ]
 
+  defines = [ "OZONE_IMPLEMENTATION" ]
+
   deps = [
     "//base",
     "//skia",