Fix build breakage from lock_guard error (#6161)

Summary:
This change fixes a source issue that caused compile time error which breaks build for many fbcode services in that setup. The size() member function of channel is a const member, so member variables accessed within it are implicitly const as well. This caused error when clang fails to resolve to a constructor that takes std::mutex because the suitable constructor got rejected due to loss of constness for its argument. The fix is to add mutable modifier to the lock_ member of channel.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6161

Differential Revision: D18967685

Pulled By: maysamyabandeh

fbshipit-source-id: 698b6a5153c3c92eeacb842c467aa28cc350d432

util/channel.h

@@ -60,7 +60,7 @@ class channel {
 
  private:
   std::condition_variable cv_;
-  std::mutex lock_;
+  mutable std::mutex lock_;
   std::queue<T> buffer_;
   bool eof_;
 };