stats: add operator= for DataWrapVec class
stats: add operator= for DataWrapVec class

gcc/g++ 4.4.7 complained about the operator= being undefined.
This changeset adds the operator.

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin