Fix compile error in tailing_iter.h

Summary:
Fix the following compile error

  ./db/tailing_iter.h:17:1: error: class 'SuperVersion' was previously declared as a struct [-Werror,-Wmismatched-tags]
  class SuperVersion;
  ^
  ./db/column_family.h:77:8: note: previous use is here
  struct SuperVersion {
         ^
         ./db/tailing_iter.h:17:1: note: did you mean struct here?
         class SuperVersion;
         ^~~~~
         struct
         1 error generated.

Test Plan: make

Reviewers: ljin, igor, haobo, sdong

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17799

db/tailing_iter.h

@@ -14,7 +14,7 @@ namespace rocksdb {
 
 class DBImpl;
 class Env;
-class SuperVersion;
+struct SuperVersion;
 class ColumnFamilyData;
 
 /**