bench: Add missing pow.h header

Fix a build error introduced in bitcoin#13219.

```
.../bitcoin/src/bench/block_assemble.cpp:42:13:error: use of undeclared identifier 'CheckProofOfWork'
    while (!CheckProofOfWork(block->GetHash(), block->nBits, Params().GetConsensus())) {
```

src/bench/block_assemble.cpp

@@ -9,6 +9,7 @@
 #include <consensus/validation.h>
 #include <miner.h>
 #include <policy/policy.h>
+#include <pow.h>
 #include <scheduler.h>
 #include <txdb.h>
 #include <txmempool.h>