Include atomic in SingletonTestStructs.cpp

Summary: This was relying on it being transitively included by system headers, which isn't the case under MSVC.

Reviewed By: yfeldblum

Differential Revision: D4199882

fbshipit-source-id: c25dc1e86687e19851923a1a660a91bcf63f0e6d

folly/test/SingletonTestStructs.cpp

@@ -16,6 +16,8 @@
 
 #include <folly/test/SingletonTestStructs.h>
 
+#include <atomic>
+
 namespace {
 // A simple class that tracks how often instances of the class and
 // subclasses are created, and the ordering.  Also tracks a global