backupable_db.cc: lambada to explictly caputre "this" when escaping scope

Summary:
Google C++ Style writes: In particular, prefer to write lambda captures explicitly when capturing this or if the lambda will escape the current scope.
Here it is the case for both.

Test Plan: Run all test suites.

Reviewers: andrewkr, dhruba

Reviewed By: andrewkr, dhruba

Subscribers: yhchiang, IslamAbdelRahman, leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D58515

utilities/backupable/backupable_db.cc

@@ -632,7 +632,7 @@ Status BackupEngineImpl::Initialize() {
   // set up threads perform copies from files_to_copy_or_create_ in the
   // background
   for (int t = 0; t < options_.max_background_operations; t++) {
-    threads_.emplace_back([&]() {
+    threads_.emplace_back([this]() {
       CopyOrCreateWorkItem work_item;
       while (files_to_copy_or_create_.read(work_item)) {
         CopyOrCreateResult result;