[aes/dv/aes_model_dpi] Include correct `string.h`
[aes/dv/aes_model_dpi] Include correct `string.h`

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan