minigbm: Update Android.mk to export the include path

minigbm: Update Android.mk to export the include path

Allows other applications depending on it to pick up minigbm headers
without explicitly specifying the path.

BUG=b:78869504
TEST=Compiled application depending on minigbm

Change-Id: Ie1681579145620ba330721393f0ad71a00109102
Reviewed-on: https://chromium-review.googlesource.com/1054615
Commit-Ready: Daniel Nicoara <dnicoara@chromium.org>
Tested-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>