Support ozone/gbm back end in standalone gn build.

This makes it possible to do a standalone gn build of the ozone/gbm back
end by using the following gn args.
  use_ozone = true
  ozone_platform_gbm = true

BUG=angleproject:1569

Change-Id: I70716b83943dae95df48b847ff8cca5e2c555c4a
Reviewed-on: https://chromium-review.googlesource.com/749867
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>

third_party/minigbm/BUILD.gn

@@ -0,0 +1,13 @@
+# Copyright 2017 The ANGLE Project Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/linux/pkg_config.gni")
+
+pkg_config("libgbm") {
+  packages = [ "gbm" ]
+}
+
+group("minigbm") {
+  public_configs = [ ":libgbm" ]
+}