bump grootfs
bump grootfs

Submodule src/grootfs 876c4256..0905cce4:
  > Use vendored files to build package in tests

Co-authored-by: Kieron Browne <c440ca4001e1bac635249757b129604e7589fc3c@pivotal.io>

cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release