Explicitly include <atomic>

Summary: Because, rather impressively, it wasn't being included transitively by anything here when compiling on Windows.

Reviewed By: yfeldblum

Differential Revision: D9565417

fbshipit-source-id: b83fa0b121d83cbeb7ff0bd79dec506b6549385e

folly/executors/test/ThreadPoolExecutorTest.cpp

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include <atomic>
 #include <memory>
 #include <thread>