fix no member named 'atexit' in namespace 'std'

Summary: Resolve issue here: https://fb.workplace.com/groups/560979627394613/permalink/1763417530484144/

Differential Revision: D21147181

fbshipit-source-id: 82acb7eb910deff28a722026d1f43bae855f2942

folly/init/Phase.cpp

@@ -17,6 +17,7 @@
 #include <folly/init/Phase.h>
 
 #include <atomic>
+#include <cstdlib>
 #include <stdexcept>
 
 namespace folly {