Update src/base58.h

Change Base58 version to allow importing of private keys.

Note: Any private keys dumped from a PPCoin daemon version prior to the commit will be in an incompatible Base58 version. After installing the new version of PPCoin, simply dump the addresses again to get private keys of the correct version.
This only affects private keys dumped directly from the PPCoin daemon. Vanitygen addresses are not affected as they are the correct version, provided they are generated with the -X 55 switch.