Fix unity build broken by #7007 (#7024)

Summary:
https://github.com/facebook/rocksdb/pull/7007 broken the unity build. Fix it by moving the const inside the function
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7024

Test Plan: make unity and see it to build.

Reviewed By: zhichao-cao

Differential Revision: D22212028

fbshipit-source-id: 5daff7383b691808164d4745ab543238502d946b

trace_replay/block_cache_tracer.cc

@@ -19,8 +19,6 @@
 namespace ROCKSDB_NAMESPACE {
 
 namespace {
-const unsigned int kCharSize = 1;
-
 bool ShouldTrace(const Slice& block_key, const TraceOptions& trace_options) {
   if (trace_options.sampling_frequency == 0 ||
       trace_options.sampling_frequency == 1) {
@@ -217,6 +215,8 @@ Status BlockCacheTraceReader::ReadAccess(BlockCacheTraceRecord* record) {
   record->block_type = trace.type;
   Slice enc_slice = Slice(trace.payload);
 
+  const unsigned int kCharSize = 1;
+
   Slice block_key;
   if (!GetLengthPrefixedSlice(&enc_slice, &block_key)) {
     return Status::Incomplete(

trace_replay/io_tracer.cc

@@ -17,11 +17,6 @@
 #include "util/string_util.h"
 
 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),
@@ -143,6 +138,7 @@ Status IOTraceReader::ReadIOOp(IOTraceRecord* record) {
   record->file_name = file_name.ToString();
   // TODO: Read below options based on file_operation.
   record->len = static_cast<size_t>(enc_slice[0]);
+  const unsigned int kCharSize = 1;
   enc_slice.remove_prefix(kCharSize);
   if (enc_slice.empty()) {
     return Status::Incomplete(