Fix compilation error (#5872)

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

table/block_based/full_filter_block.cc

@@ -16,9 +16,9 @@
 namespace rocksdb {
 
 FullFilterBlockBuilder::FullFilterBlockBuilder(
-    const SliceTransform* prefix_extractor, bool whole_key_filtering,
+    const SliceTransform* _prefix_extractor, bool whole_key_filtering,
     FilterBitsBuilder* filter_bits_builder)
-    : prefix_extractor_(prefix_extractor),
+    : prefix_extractor_(_prefix_extractor),
       whole_key_filtering_(whole_key_filtering),
       last_whole_key_recorded_(false),
       last_prefix_recorded_(false),

table/block_based/partitioned_filter_block.cc

@@ -18,12 +18,12 @@
 namespace rocksdb {
 
 PartitionedFilterBlockBuilder::PartitionedFilterBlockBuilder(
-    const SliceTransform* prefix_extractor, bool whole_key_filtering,
+    const SliceTransform* _prefix_extractor, bool whole_key_filtering,
     FilterBitsBuilder* filter_bits_builder, int index_block_restart_interval,
     const bool use_value_delta_encoding,
     PartitionedIndexBuilder* const p_index_builder,
     const uint32_t partition_size)
-    : FullFilterBlockBuilder(prefix_extractor, whole_key_filtering,
+    : FullFilterBlockBuilder(_prefix_extractor, whole_key_filtering,
                              filter_bits_builder),
       index_on_filter_block_builder_(index_block_restart_interval,
                                      true /*use_delta_encoding*/,