inputs: restore "const" attribute removed by D33759

Summary:
The "const" attribute applies to the type, and placing it
before that return type retains the desired semantics,
yet avoids the compiler error/warning.

Test Plan: Run make

Reviewers: ljin, sdong, igor.sugak, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D33789

db/compaction.h

@@ -92,7 +92,7 @@ class Compaction {
   // input level.
   // REQUIREMENT: "compaction_input_level" must be >= 0 and
   //              < "input_levels()"
-  std::vector<FileMetaData*>* inputs(size_t compaction_input_level) {
+  const std::vector<FileMetaData*>* inputs(size_t compaction_input_level) {
     assert(compaction_input_level < inputs_.size());
     return &inputs_[compaction_input_level].files;
   }