Update definition for GetCompactionReasonString

Summary:
`GetCompactionReasonString()` will be exposed in
RocksDB 8.7, which includes PR facebook/rocksdb#11778 that exposes it. So update its definition here to be based on RocksDB version.

Differential Revision: D49755853

fbshipit-source-id: 1ff11f6708768b0039f88835e44272f525798ba2

storage/rocksdb/rdb_i_s.cc

@@ -932,6 +932,7 @@ static int rdb_i_s_compact_stats_fill_table(
   DBUG_RETURN(ret);
 }
 
+#if ROCKSDB_MAJOR < 8 || (ROCKSDB_MAJOR == 8 && ROCKSDB_MINOR < 7)
 namespace {
 
 using rocksdb::CompactionReason;
@@ -982,6 +983,7 @@ const char *GetCompactionReasonString(CompactionReason compaction_reason) {
 }
 
 }  // anonymous namespace
+#endif
 
 /*
   Support for INFORMATION_SCHEMA.ROCKSDB_ACTIVE_COMPACTION_STATS dynamic table