Fix an error on GCC 4.8.5 where -Werror=unused-parameter fails (#9144)

Summary:
Before this fix compilation with GCC 4.8.5 20150623 (Red Hat 4.8.5-36) would fail with the following error:
```
CC       jls/db/db_impl/db_impl.o
In file included from ./env/file_system_tracer.h:8:0,
                 from ./file/random_access_file_reader.h:15,
                 from ./file/file_prefetch_buffer.h:15,
                 from ./table/format.h:13,
                 from ./table/internal_iterator.h:14,
                 from ./db/pinned_iterators_manager.h:12,
                 from ./db/range_tombstone_fragmenter.h:15,
                 from ./db/memtable.h:22,
                 from ./db/memtable_list.h:16,
                 from ./db/column_family.h:17,
                 from ./db/db_impl/db_impl.h:22,
                 from db/db_impl/db_impl.cc:9:
./include/rocksdb/file_system.h:108:8: error: unused parameter 'opts'
[-Werror=unused-parameter]
 struct FileOptions : EnvOptions {
        ^
db/db_impl/db_impl.cc: In member function 'virtual rocksdb::Status
rocksdb::DBImpl::SetDBOptions(const
std::unordered_map<std::basic_string<char>, std::basic_string<char>
>&)':
db/db_impl/db_impl.cc:1230:36: note: synthesized method
'rocksdb::FileOptions& rocksdb::FileOptions::operator=(const
rocksdb::FileOptions&)' first required here
       file_options_for_compaction_ = FileOptions(new_db_options);
                                    ^
  CC       jls/db/db_impl/db_impl_compaction_flush.o
cc1plus: all warnings being treated as errors
make[1]: *** [jls/db/db_impl/db_impl.o] Error 1
make[1]: *** Waiting for unfinished jobs....
make[1]: Leaving directory `/rocksdb-local-build'
make: *** [rocksdbjavastatic] Error 2
Makefile:2222: recipe for target 'rocksdbjavastaticdockerarm64v8' failed
make: *** [rocksdbjavastaticdockerarm64v8] Error 2
```

This was detected on both ppc64le and arm64v8, however it does not seem to appear in the same GCC 4.8 version we use for x64 in CircleCI - https://app.circleci.com/pipelines/github/facebook/rocksdb/9691/workflows/c2a94367-14f3-4039-be95-325c34643d41/jobs/227906

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9144

Reviewed By: riversand963

Differential Revision: D32290770

fbshipit-source-id: c90a54ba2a618e1ff3660fff3f3368ab36c3c527

include/rocksdb/file_system.h

@@ -165,7 +165,7 @@ struct FileOptions : EnvOptions {
         temperature(opts.temperature),
         handoff_checksum_type(opts.handoff_checksum_type) {}
 
-  FileOptions& operator=(const FileOptions& opts) = default;
+  FileOptions& operator=(const FileOptions&) = default;
 };
 
 // A structure to pass back some debugging information from the FileSystem