Fix variable shadowing in rpc/mining.cpp
Fix variable shadowing in rpc/mining.cpp

rpc/mining.cpp:661:14: warning: declaration shadows a local variable [-Wshadow]
    for (int i = 0; i < (int)Consensus::MAX_VERSION_BITS_DEPLOYMENTS; ++i)
             ^
rpc/mining.cpp:614:9: note: previous declaration is here
    int i = 0;
        ^
rpc/mining.cpp:976:43: warning: declaration shadows a variable in the global namespace [-Wshadow]
void RegisterMiningRPCCommands(CRPCTable &tableRPC)
                                          ^
./rpc/server.h:176:18: note: previous declaration is here
extern CRPCTable tableRPC;
                 ^

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