Fix struct/class declaration mismatches

src/miner.h

@@ -10,7 +10,7 @@
 
 class CBlock;
 class CBlockIndex;
-class CBlockTemplate;
+struct CBlockTemplate;
 class CReserveKey;
 class CScript;
 class CWallet;

src/walletdb.h

@@ -16,7 +16,7 @@
 
 class CAccount;
 class CAccountingEntry;
-class CBlockLocator;
+struct CBlockLocator;
 class CKeyPool;
 class CMasterKey;
 class CScript;