drm-tests: Fix warnings in null_platform_test

Fix some missing-prototypes and strict-prototypes warnings.

BUG=None
TEST=Local build with -Wall is now free of warnings

Change-Id: Ic892ea3bfcc35589bdc108eee744030468040e4d
Reviewed-on: https://chromium-review.googlesource.com/236740
Reviewed-by: Lauri Peltonen <lpeltonen@nvidia.com>
Commit-Queue: Lauri Peltonen <lpeltonen@nvidia.com>
Tested-by: Lauri Peltonen <lpeltonen@nvidia.com>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
1 file changed