Summary:
Without this fix, compiler complains.
```
$ROCKSDB_NO_FBCODE=1 USE_CLANG=1 make ldb
table/block_based/full_filter_block.cc: In constructor ‘rocksdb::FullFilterBlockBuilder::FullFilterBlockBuilder(const rocksdb::SliceTransform*, bool, rocksdb::FilterBitsBuilder*)’:
table/block_based/full_filter_block.cc:20:43: error: declaration of ‘prefix_extractor’ shadows a member of 'this' [-Werror=shadow]
FilterBitsBuilder* filter_bits_builder)
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5872

Test Plan:
```
$ROCKSDB_NO_FBCODE=1 make all
```

Differential Revision: D17690058

Pulled By: riversand963

fbshipit-source-id: 19e3d9bd86e1123847095240e73d30da5d66240e