Fixed unix makefile to include boost::chrono

Compiling on Linux fails due to missing links in makefile.unix

```
$ make -f makefile.unix 
/bin/sh ../share/genbuild.sh obj/build.h
g++ -O2 -pthread -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter  -DBOOST_SPIRIT_THREADSAFE -D_FILE_OFFSET_BITS=64 -I/home/afri/opt/primecoin-pool/src -I/home/afri/opt/primecoin-pool/src/obj -DUSE_IPV6=1 -I/home/afri/opt/primecoin-pool/src/leveldb/include -I/home/afri/opt/primecoin-pool/src/leveldb/helpers -DHAVE_BUILD_INFO -fno-stack-protector -fstack-protector-all -Wstack-protector -D_FORTIFY_SOURCE=2  -o primeminer leveldb/libleveldb.a obj/alert.o obj/version.o obj/checkpoints.o obj/netbase.o obj/addrman.o obj/crypter.o obj/key.o obj/db.o obj/keystore.o obj/main.o obj/net.o obj/protocol.o obj/bitcoinrpc.o obj/rpcdump.o obj/rpcnet.o obj/rpcmining.o obj/rpcwallet.o obj/rpcblockchain.o obj/rpcrawtransaction.o obj/script.o obj/sync.o obj/util.o obj/wallet.o obj/walletdb.o obj/hash.o obj/bloom.o obj/noui.o obj/leveldb.o obj/txdb.o obj/prime.o obj/checkpointsync.o obj/main_poolminer.o -Wl,-z,relro -Wl,-z,now   -Wl,-Bdynamic -l boost_system -l boost_filesystem -l boost_program_options -l boost_thread -l db_cxx -l ssl -l crypto -l rt -Wl,-Bdynamic -l gmp -Wl,-Bdynamic -Wl,-Bdynamic -l z -l dl -l pthread /home/afri/opt/primecoin-pool/src/leveldb/libleveldb.a /home/afri/opt/primecoin-pool/src/leveldb/libmemenv.a
obj/db.o: In function `CDB::Rewrite(std::string const&, char const*)':
db.cpp:(.text+0x2a14): undefined reference to `boost::chrono::system_clock::now()'
db.cpp:(.text+0x2a1c): undefined reference to `boost::chrono::steady_clock::now()'
db.cpp:(.text+0x2bf8): undefined reference to `boost::chrono::steady_clock::now()'
obj/main.o: In function `boost::this_thread::sleep_for(boost::chrono::duration<long, boost::ratio<1l, 1000000000l> > const&)':
main.cpp:(.text._ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE[_ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE]+0x5a): undefined reference to `boost::chrono::system_clock::now()'
main.cpp:(.text._ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE[_ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE]+0x62): undefined reference to `boost::chrono::steady_clock::now()'
main.cpp:(.text._ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE[_ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE]+0xa8): undefined reference to `boost::chrono::steady_clock::now()'
obj/walletdb.o: In function `ThreadFlushWalletDB(std::string const&)':
walletdb.cpp:(.text+0x1a75): undefined reference to `boost::chrono::system_clock::now()'
walletdb.cpp:(.text+0x1a7d): undefined reference to `boost::chrono::steady_clock::now()'
walletdb.cpp:(.text+0x1b86): undefined reference to `boost::chrono::steady_clock::now()'
collect2: error: ld returned 1 exit status
make: *** [primeminer] Error 1
```

Fixes to makefile applied and confirmed working now :>