Add missing include for `abort()`

Summary:
Fixes #1233 (again).
Closes https://github.com/facebook/rocksdb/pull/1931

Differential Revision: D4625289

Pulled By: ajkr

fbshipit-source-id: 70e774e

util/threadpool_imp.cc

@@ -24,6 +24,7 @@
 #include <atomic>
 #include <condition_variable>
 #include <mutex>
+#include <stdlib.h>
 #include <thread>
 #include <vector>