Fix double define in IO_tracer (#7007)

Summary:
Fix the following error

"./trace_replay/io_tracer.h:20:20: error: redefinition of ‘const unsigned int rocksdb::{anonymous}::kCharSize’
 const unsigned int kCharSize = 1;
                    ^~~~~~~~~
In file included from unity.cc:177:
trace_replay/block_cache_tracer.cc:22:20: note: ‘const unsigned int rocksdb::{anonymous}::kCharSize’ previously defined here
 const unsigned int kCharSize = 1;"
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7007

Reviewed By: akankshamahajan15

Differential Revision: D22142618

Pulled By: zhichao-cao

fbshipit-source-id: e6dcd51ccc21d1f58df52cdc7a1c88e54cf4f6e8

trace_replay/io_tracer.cc

@@ -18,6 +18,10 @@
 
 namespace ROCKSDB_NAMESPACE {
 
+namespace {
+const unsigned int kCharSize = 1;
+}  // namespace
+
 IOTraceWriter::IOTraceWriter(Env* env, const TraceOptions& trace_options,
                              std::unique_ptr<TraceWriter>&& trace_writer)
     : env_(env),

trace_replay/io_tracer.h

@@ -16,10 +16,6 @@
 
 namespace ROCKSDB_NAMESPACE {
 
-namespace {
-const unsigned int kCharSize = 1;
-}  // namespace
-
 struct IOTraceRecord {
   // Required fields for all accesses.
   uint64_t access_timestamp = 0;