refactor: Move GetDifficulty out of rpc/server.h

It has no business in `rpcserver.h`. Define it in the interface header
of the implementation unit `rpcblockchain` where it is defined.

Also modernize the signature to:

    double GetDifficulty(const CBlockIndex* blockindex = nullptr);

(remove `extern`, replace `NULL` with `nullptr`)

src/rpc/mining.cpp

@@ -16,6 +16,7 @@
 #include "miner.h"
 #include "net.h"
 #include "pow.h"
+#include "rpc/blockchain.h"
 #include "rpc/server.h"
 #include "txmempool.h"
 #include "util.h"