Fix compile error (#7908)

Summary:
OS: Ubuntu 14.04
Compiler: GCC 4.9.4
Compile error:
```
db/forward_iterator.cc:996:62: error: declaration of ‘key’ shadows a member of 'this' [-Werror=shadow]
   auto cmp = [&](const FileMetaData* f, const Slice& key) -> bool {
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7908

Reviewed By: jay-zhuang

Differential Revision: D26899986

Pulled By: ajkr

fbshipit-source-id: 66b0b97aefd0f13a085e063491f8207366a9f848

db/forward_iterator.cc

@@ -993,9 +993,9 @@ bool ForwardIterator::TEST_CheckDeletedIters(int* pdeleted_iters,
 uint32_t ForwardIterator::FindFileInRange(
     const std::vector<FileMetaData*>& files, const Slice& internal_key,
     uint32_t left, uint32_t right) {
-  auto cmp = [&](const FileMetaData* f, const Slice& key) -> bool {
+  auto cmp = [&](const FileMetaData* f, const Slice& k) -> bool {
     return cfd_->internal_comparator().InternalKeyComparator::Compare(
-            f->largest.Encode(), key) < 0;
+            f->largest.Encode(), k) < 0;
   };
   const auto &b = files.begin();
   return static_cast<uint32_t>(std::lower_bound(b + left,