Bluetooth: Mesh: Fixed OP_AGG_SRV dependency of OP_AGG_CLI

The OP_AGG_SRV is dependent that the OP_AGG_CLI is add
for the OP_AGG_SRV to work. Moved the define of OP_AGG_CLI
so it will call op_agg_send without the OP_AGG_CLI.

Signed-off-by: Ingar Kulbrandstad <ingar.kulbrandstad@nordicsemi.no>