Wladimir J. van der Laan cec84c2f1a bench: Add missing pow.h header
Fix a build error introduced in #13219.

```
.../bitcoin/src/bench/block_assemble.cpp:42:13:error: use of undeclared identifier 'CheckProofOfWork'
    while (!CheckProofOfWork(block->GetHash(), block->nBits, Params().GetConsensus())) {
```
2018-06-24 19:25:03 +02:00
..
2015-10-06 17:46:12 +02:00
2018-04-18 19:53:10 -07:00
2018-05-20 09:15:39 +02:00