Add _GNU_SOURCE to prebuilt-intermediates/config.h am: c192305856 am: 39c3ff5af0 am: 2919cb9f36 am: 00a5462fab

Original change: https://android-review.googlesource.com/c/platform/external/virglrenderer/+/2237835

Change-Id: I76f616a21af739add31fb73fc475ecaac395cd25
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>