drm_hwcomposer: Use -Werror in external/drm_hwcomposer

* Suppress non-critical warnings.

I'm submitting this patch to try to align aosp/master
with the upstream fdo/master branch.

Bug: 66996870
Change-Id: I48e87526e408980c90e8a45e134f95da7a89a19d
Signed-off-by: John Stultz <john.stultz@linaro.org>
1 file changed