Fix clang build failure with -Wgnu-redeclared-enum

Summary:
In include/rocksdb/db.h, enum EntryType is redeclared even though
original declaration in types.h in included.
Closes https://github.com/facebook/rocksdb/pull/3766

Differential Revision: D7765504

Pulled By: anand1976

fbshipit-source-id: 622a8ecb306993915be1b9dd5cdd79dbc6a4ea05

include/rocksdb/db.h

@@ -53,7 +53,6 @@ struct ExternalSstFileInfo;
 class WriteBatch;
 class Env;
 class EventListener;
-enum EntryType;
 
 using std::unique_ptr;