fix compilation error (same as fix #284)

[maa@srv2-nskb-devg2 rocksdb-master]$ CXX=/usr/local/CC/gcc-4.7.4/bin/g++ EXTRA_CXXFLAGS=-std=c++11 DISABLE_WARNING_AS_ERROR=1  make db_bench
  CC       db/db_bench.o
db/db_bench.cc: In member function 'rocksdb::Slice rocksdb::Benchmark::AllocateKey(std::unique_ptr<const char []>*)':
db/db_bench.cc:1434:41: error: use of deleted function 'void std::unique_ptr<_Tp [], _Dp>::reset(_Up) [with _Up = char*; _Tp = const char; _Dp = std::default_delete<const char []>]'
In file included from /usr/local/CC/gcc-4.7.4/lib/gcc/x86_64-unknown-linux-gnu/4.7.4/../../../../include/c++/4.7.4/memory:86:0,
                 from ./include/rocksdb/db.h:14,
                 from ./db/dbformat.h:14,
                 from ./db/db_impl.h:21,
                 from db/db_bench.cc:33:

db/db_bench.cc

@@ -1431,7 +1431,9 @@ class Benchmark {
   }
 
   Slice AllocateKey(std::unique_ptr<const char[]>* key_guard) {
-    key_guard->reset(new char[key_size_]);
+    char* data = new char[key_size_];
+    const char* const_data = data;
+    key_guard->reset(const_data);
     return Slice(key_guard->get(), key_size_);
   }