Fix linking

When compiling on Ubuntu 12.10, x86_64, g++ 4.7.2, I get this linking error:
```
riba.o: In function `(anonymous namespace)::clear_db_state()':
riba.cpp:(.text+0x28): undefined reference to `leveldb::WriteBatch::Clear()'
riba.o: In function `leveldb_open(char const*)':
riba.cpp:(.text+0x831): undefined reference to `leveldb::Options::Options()'
riba.cpp:(.text+0x888): undefined reference to `leveldb::DB::Open(leveldb::Options const&, std::string const&, leveldb::DB**)'
riba.cpp:(.text+0x8cc): undefined reference to `leveldb::Status::ToString() const'
riba.o: In function `leveldb_get(char const*)':
riba.cpp:(.text+0xb0a): undefined reference to `leveldb::Status::ToString() const'
riba.o: In function `leveldb_put(char const*, char const*)':
riba.cpp:(.text+0xc51): undefined reference to `leveldb::WriteBatch::Put(leveldb::Slice const&, leveldb::Slice const&)'
riba.cpp:(.text+0xcb8): undefined reference to `leveldb::Status::ToString() const'
riba.o: In function `leveldb_delete(char const*)':
riba.cpp:(.text+0xdb2): undefined reference to `leveldb::WriteBatch::Delete(leveldb::Slice const&)'                                                                                                                                                                            
riba.cpp:(.text+0xe12): undefined reference to `leveldb::Status::ToString() const'                                                                                                                                                                                             
riba.o: In function `leveldb_start_batch()':                                                                                                                                                                                                                                   
riba.cpp:(.text+0xec0): undefined reference to `leveldb::WriteBatch::Clear()'                                                                                                                                                                                                  
riba.o: In function `leveldb_commit_batch()':                                                                                                                                                                                                                                  
riba.cpp:(.text+0xf4c): undefined reference to `leveldb::Status::ToString() const'                                                                                                                                                                                             
riba.cpp:(.text+0xf94): undefined reference to `leveldb::WriteBatch::Clear()'                                                                                                                                                                                                  
riba.o: In function `__static_initialization_and_destruction_0(int, int)':                                                                                                                                                                                                     
riba.cpp:(.text+0x170c): undefined reference to `leveldb::WriteBatch::WriteBatch()'                                                                                                                                                                                            
riba.cpp:(.text+0x171b): undefined reference to `leveldb::WriteBatch::~WriteBatch()'                                                                                                                                                                                           
main.o: In function `main':                                                                                                                                                                                                                                                    
main.cpp:(.text+0x2a): undefined reference to `rl_insert'                                                                                                                                                                                                                      
main.cpp:(.text+0x34): undefined reference to `rl_bind_key'                                                                                                                                                                                                                    
main.cpp:(.text+0x3e): undefined reference to `readline'                                                                                                                                                                                                                       
main.cpp:(.text+0xef): undefined reference to `add_history'       
```
This pull request fixes it by reversing the order of the g++ flags.