[mining] segwit option must be set in GBT

Calling getblocktemplate without the segwit rule specified is most
likely a client error, since it results in lower fees for the miner.
Prevent this client error by failing getblocktemplate if called without
the segwit rule specified.

src/miner.h

@@ -157,7 +157,7 @@ public:
     BlockAssembler(const CChainParams& params, const Options& options);
 
     /** Construct a new block template with coinbase to scriptPubKeyIn */
-    std::unique_ptr<CBlockTemplate> CreateNewBlock(const CScript& scriptPubKeyIn, bool fMineWitnessTx=true);
+    std::unique_ptr<CBlockTemplate> CreateNewBlock(const CScript& scriptPubKeyIn);
 
 private:
     // utility functions