Fix data race in propagateRROEnforcementMutator

propagateRROEnforcementMutator walks dependencies of each module and
modifies them via SetRROEnforcedForDependent.  That is unsafe in a
parallel mutator, as the same dependency could be being visited by
two parent modules at the same time.  Don't mark it as a parallel
mutator.

Test: go test -race ./...
Change-Id: I9870aaa7d165bf72b751e3ceff5a2a3eb4d74ec4