SyncWAL shouldn't be supported in compacted db (#7788)

Summary:
`CompactedDB` is a kind of read-only DB, so it shouldn't support `SyncWAL`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7788

Test Plan: watch existing tests to pass.

Reviewed By: akankshamahajan15

Differential Revision: D25661209

Pulled By: cheng-chang

fbshipit-source-id: 9eb2cc3f73736dcc205c8410e5944aa203f002d3

db/compacted_db_impl.h

@@ -82,6 +82,11 @@ class CompactedDBImpl : public DBImpl {
                        ColumnFamilyHandle* /*column_family*/) override {
     return Status::NotSupported("Not supported in compacted db mode.");
   }
+
+  virtual Status SyncWAL() override {
+    return Status::NotSupported("Not supported in compacted db mode.");
+  }
+
   using DB::IngestExternalFile;
   virtual Status IngestExternalFile(
       ColumnFamilyHandle* /*column_family*/,