Berkeley DB v6 compatibility fix

Fixes building error looking like this:
CXX wallet/libbitcoin_wallet_a-db.o
wallet/db.cpp: In member function ‘void CDBEnv::EnvShutdown()’:
wallet/db.cpp:46:16: error: call of overloaded ‘DbEnv(int)’ is ambiguous
DbEnv(0).remove(strPath.c_str(), 0);
^
wallet/db.cpp:46:16: note: candidates are:
In file included from wallet/db.h:21:0,
from wallet/db.cpp:6:
/usr/include/db_cxx.h:916:2: note: DbEnv::DbEnv(const DbEnv&)
DbEnv(const DbEnv &);
^
/usr/include/db_cxx.h:518:2: note: DbEnv::DbEnv(DB_ENV)
DbEnv(DB_ENV *dbenv);
^
/usr/include/db_cxx.h:516:2: note: DbEnv::DbEnv(u_int32_t)
DbEnv(u_int32_t flags);
^
Makefile:5780: recipe for target 'wallet/libbitcoin_wallet_a-db.o' failed
make[2]: ** [wallet/libbitcoin_wallet_a-db.o] Error 1

src/wallet/db.cpp

@@ -43,7 +43,7 @@ void CDBEnv::EnvShutdown()
     if (ret != 0)
         LogPrintf("CDBEnv::EnvShutdown: Error %d shutting down database environment: %s\n", ret, DbEnv::strerror(ret));
     if (!fMockDb)
-        DbEnv(0).remove(strPath.c_str(), 0);
+        DbEnv((u_int32_t)0).remove(strPath.c_str(), 0);
 }
 
 void CDBEnv::Reset()